Add X clipboard support layer to terminal emacs.

The X clipboard allows a user to copy and paste content between different X
windows, e.g. copying text from Chrome into a Terminal. Copy/pasting with the X
clipboard it well supported in GUI Emacs, but not so well in terminal Emacs
(i.e `emacs -nw` or `emacsclient -t`) without resorting to using the mouse,
since terminal Emacs has no awareness of X. There are several incomplete Elisp
solutions out that work for the most part, but may not have cross-platform
support, or may fail over SSH with X forwarding or within a `tmux` session.

This layer adds support for OSX, Linux, Windows, and Cygwin using the relevant
binary on each system. For example on Linux, it uses `xsel` or `xclip` to
interface with the clipboard, depending which one is available. It also adds
support for ssh'ing into a different OS with X forwarding via `ssh -Y hostname`,
and copy/pasting to and from a remote terminal Emacs. It also supports an edge
case of continuing to work in an Emacs instance running inside a `tmux` session
which may have been started by a different ssh session, which relies on
explicitly reseting the `$DISPLAY` environment variable before calling the
relevant binary.

Yank code inspired by https://github.com/tmux-plugins/tmux-yank.

Fix #4662.

Tried to add xclip support but removed it since it freezes.

layers/+tools/xclipboard/README.org

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+#+TITLE: xclipboard layer
+
+* Table of Contents                                        :TOC_4_gh:noexport:
+ - [[#description][Description]]
+ - [[#key-bindings][Key Bindings]]
+
+* Description
+This layer adds copy/paste support to the X-clipboard from the terminal.
+
+* Key Bindings
+
+| Key Binding | Description                                 |
+|-------------+---------------------------------------------|
+| ~SPC x y~   | Copy selection to clipboard                 |
+| ~SPC x p~   | Paste clipboard contents at cursor position |

layers/+tools/xclipboard/funcs.el

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+;;; funcs.el --- xclipboard layer functions file for Spacemacs.
+;;
+;; Copyright (c) 2012-2018 Sylvain Benner & Contributors
+;;
+;; Authors: Charles Weill <weill@google.com>
+;;          Google LLC.
+;; URL: https://github.com/syl20bnr/spacemacs
+;;
+;; This file is not part of GNU Emacs.
+;;
+;;; License: GPLv3
+
+(defun spacemacs//xclipboard-get-display ()
+  (shell-command-to-string "if [[ -n $TMUX ]]; then
+      export DISPLAY=$(tmux show-environment | grep -o '^DISPLAY.*$' | sed 's/DISPLAY=//')
+    fi
+    if [[ -z $DISPLAY ]]; then
+      export DISPLAY=:0
+    fi
+    printf $DISPLAY")
+  )
+
+(defun spacemacs//xclipboard-get-copy-command ()
+  (shell-command-to-string "command_exists() {
+      local command=\"$1\"
+      type \"$command\" >/dev/null 2>&1
+    }
+
+    # Installing reattach-to-user-namespace is recommended on OS X
+    if command_exists \"pbcopy\"; then
+        if command_exists \"reattach-to-user-namespace\"; then
+            printf \"reattach-to-user-namespace pbcopy\"
+        else
+            printf \"pbcopy\"
+        fi
+    elif command_exists \"clip.exe\"; then # WSL clipboard command
+        printf \"clip.exe\"
+    elif command_exists \"xsel\"; then
+        printf \"xsel -ib\"
+    elif command_exists \"putclip\"; then # cygwin clipboard command
+        printf \"putclip\"
+    fi")
+  )
+
+(defun spacemacs//xclipboard-get-paste-command ()
+  (shell-command-to-string "command_exists() {
+      local command=\"$1\"
+      type \"$command\" >/dev/null 2>&1
+    }
+
+    # Installing reattach-to-user-namespace is recommended on OS X
+    if command_exists \"pbpaste\"; then
+        if command_exists \"reattach-to-user-namespace\"; then
+            printf \"reattach-to-user-namespace pbpaste\"
+        else
+            printf \"pbpaste\"
+        fi
+    elif command_exists \"paste.exe\"; then # WSL clipboard command
+        printf \"paste.exe\"
+    elif command_exists \"xsel\"; then
+        printf \"xsel -ob\"
+    elif command_exists \"getclip\"; then # cygwin clipboard command
+        printf \"getclip\"
+    fi")
+  )
+
+(defun spacemacs/xclipboard-copy ()
+  "Copies selection to x-clipboard."
+  (interactive)
+  (if (display-graphic-p)
+    (progn
+      (message "Copied region to x-clipboard!")
+      (call-interactively 'clipboard-kill-ring-save)
+      )
+    (if (region-active-p)
+      (progn
+        (shell-command-on-region (region-beginning) (region-end) (format "DISPLAY=%s %s" (spacemacs//xclipboard-get-display) (spacemacs//xclipboard-get-copy-command)))
+        (message (format "Copied region to clipboard \"%s\"!" (spacemacs//xclipboard-get-display)))
+        (deactivate-mark)
+        )
+      (message "No region active; can't copy to clipboard!")
+      )
+    )
+  )
+
+(defun spacemacs/xclipboard-paste ()
+  "Pastes from x-clipboard."
+  (interactive)
+  (if (display-graphic-p)
+    (progn
+      (clipboard-yank)
+      (message "graphics active")
+      )
+    (insert (shell-command-to-string (format "DISPLAY=%s %s" (spacemacs//xclipboard-get-display) (spacemacs//xclipboard-get-paste-command))))
+    )
+  (message (format "Pasted from clipboard \"%s\"!" (spacemacs//xclipboard-get-display)))
+  )

layers/+tools/xclipboard/keybindings.el

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+;;; keybindings.el --- xclipboard layer keybindings file for Spacemacs.
+;;
+;; Copyright (c) 2012-2018 Sylvain Benner & Contributors
+;;
+;; Authors: Charles Weill <weill@google.com>
+;;          Google LLC.
+;; URL: https://github.com/syl20bnr/spacemacs
+;;
+;; This file is not part of GNU Emacs.
+;;
+;;; License: GPLv3
+
+(evil-leader/set-key "x y" 'spacemacs/xclipboard-copy)
+(evil-leader/set-key "x p" 'spacemacs/xclipboard-paste)

layers/+tools/xclipboard/packages.el

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+;;; packages.el --- xclipboard layer packages file for Spacemacs.
+;;
+;; Copyright (c) 2012-2018 Sylvain Benner & Contributors
+;;
+;; Authors: Charles Weill <weill@google.com>
+;;          Google LLC.
+;; URL: https://github.com/syl20bnr/spacemacs
+;;
+;; This file is not part of GNU Emacs.
+;;
+;;; License: GPLv3
+
+(defconst xclipboard-packages '())
+
+(defun xclipboard/init-xclipboard ()
+  (use-package xclipboard))