Proposal: Make the xclipboard layer to use system clipboard for copy/paste

[xuhdev]

Currently, the xclipboard layer provides spacemacs/xclipboard-copy and
spacemacs/xclipboard-paste for copying and pasting the active region. But I think
things can become more useful if we make the layer to copy/paste from the
clipboard by default with a function to switch it on/off, for all kinds of
copying/pasting.

I have a small script that works with wayland: https://gist.github.com/yorickvP/6132f237fbc289a45c808d8d75e0e1fb?permalink_comment_id=5345689#gistcomment-5345689

In a nutshell, it replaces interprogram-cut-function and
interprogram-paste-function to make use of the external clipboard program. It
should be easily adaptable to other commands.

What do you think?