Customize `*exwm-edit*` buffer placement with `display-buffer-action`

[josephmturner]

Thank you for exwm-edit!!

Here's some extra explanation for the two main commits:

• 3943f88 replaces the globalized minor mode with a simpler, more robust way to ensure that exwm-edit-mode-map bindings are not shadowed when switching major mode.
• e02d2f5 replaces exwm-edit-split with exwm-edit-display-buffer-action, allowing greater flexibility in where *exwm-edit XXX * buffers are opened.

Since I submitted many PRs, I'm happy to rebase my work if need be.

Thanks,

Joseph