Add exwm-minor-mode

[progfolio]

Previous pull: https://github.com/ch11ng/exwm/pull/868

@medranocalvo Has already given blessing for someone to pick up this work.

There was some discussion regarding restoring a replaced window manager when exwm-mode is disabled. My suggestion would be to add a function to exwm-exit-functions which covers the cases we know we can replace easily and leave it up to the user to add such a function for cases we can't predict or reliably restore. Regardless, I think moving to a mode and away from "exwm-enable" is important.