Add imenu-list layer

imenu-list provides a side bar that shows all imenu entries of the
current buffer

layers/+tools/imenu-list/README.org

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+* imenu-list contribution layer for Spacemacs
+** Table of Contents                                                  :TOC@4:
+ - [[#imenu-list-contribution-layer-for-spacemacs][imenu-list contribution layer for Spacemacs]]
+   - [[#description][Description]]
+   - [[#install][Install]]
+   - [[#key-bindings][Key bindings]]
+     - [[#from-any-buffer][From any buffer]]
+     - [[#from-imenu-list-buffer][From imenu-list buffer]]
+
+** Description
+This layer uses [[https://github.com/bmag/imenu-list][imenu-list]] to show the current buffer's index in a side bar.
+[[file:img/imenu-list-example.png]]
+
+** Install
+To use this contribution add it to your =~/.spacemacs=
+
+#+begin_src emacs-lisp
+  (setq-default dotspacemacs-configuration-layers '(imenu-list))
+#+end_src
+
+** Key bindings
+*** From any buffer
+| Key Binding | Description              |
+|-------------+--------------------------|
+| ~SPC b i~   | toggle imenu-list window |
+*** From imenu-list buffer
+| Key Binding | Description                                            |
+|-------------+--------------------------------------------------------|
+| ~q~         | quit imenu-list window                                 |
+| ~RET~       | go to current entry                                    |
+| ~d~         | display current entry, keep focus on imenu-list window |
+| ~f~         | fold/unfold current section                            |

layers/+tools/imenu-list/packages.el

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+;;; packages.el --- imenu-list Layer packages File for Spacemacs
+;;
+;; Copyright (c) 2012-2014 Sylvain Benner
+;; Copyright (c) 2014-2015 Sylvain Benner & Contributors
+;;
+;; Author: Sylvain Benner <sylvain.benner@gmail.com>
+;; URL: https://github.com/syl20bnr/spacemacs
+;;
+;; This file is not part of GNU Emacs.
+;;
+;;; License: GPLv3
+
+;; List of all packages to install and/or initialize. Built-in packages
+;; which require an initialization must be listed explicitly in the list.
+(setq imenu-list-packages '(imenu-list))
+
+;; List of packages to exclude.
+(setq imenu-list-excluded-packages '())
+
+(defun imenu-list/init-imenu-list ()
+  (use-package imenu-list
+    :defer t
+    :init
+    (progn
+      (setq imenu-list-focus-after-activation t
+            imenu-list-auto-resize t)
+      (spacemacs/set-leader-keys "bi" #'imenu-list-minor-mode))
+    :config
+    (evilified-state-evilify-map imenu-list-major-mode-map
+      :mode imenu-list-major-mode
+      :bindings
+      "d" #'imenu-list-display-entry
+      "q" #'imenu-list-minor-mode)))