When requesting to a take-focus window to acquire the input, the client may or
may not respond with a SetInputFocus (this doesn't happen for no-input gtk
windows in fact [to be fixed there too]), in such case we were unsetting the
focus while waiting the reply.
In case the client won't respond, we wait for a small delay (set to 250 ms) for
the take-focus window to grab the input focus before setting it to the default
window.
Added a test for this behavior and for the case in which a window takes the
focus meanwhile we're waiting to focus the default window.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/307
