New function `dotspacmacs/emacs-custom-settings` wrapping Emacs
custom settings sexps.
`dotspacemacs/emacs-custom-settings` is called just after the user
configuration (`dotspacemacs/user-config`)
Customize cannot write its auto-generated sexps inside a function, to
accomplish this we trick Emacs by setting the custom file to a file
in `.cache` directory, the path to this file is defined by the variable
`spacemacs--custom-file`. At the startup of Emacs we read this file
to insert its content inside the function
`dotspacemacs/emacs-custom-settings` in the dotfile, this is done in the
function `spacemacs/write-custom-settings-to-dotfile`.
I don't think we need to write the custom settings to the dotfile when
exiting Emacs as well, since we do it at startup at the very beginning
(i.e. before actually loading the dotfile) we should be OK.
Fixes#5170
e699f1816a