display: (Optionally) delay focus changes in focus-follows-mouse mode

Moving focus immediately on crossing events as we currently do
in focus-follows-mouse mode may trigger a lot of unwanted focus
changes when moving over unrelated windows on the way to a target.
Those accidental focus changes prevent features like GNOME Shell's
application menu from working properly and are visually expensive
since we now use a very distinct style for unfocused windows.
Instead, delay the actual focus change until the pointer has stopped
moving.

https://bugzilla.gnome.org/show_bug.cgi?id=678169