As per commit f71151a5 we were ignoring WM_TAKE_FOCUS-only windows as focus
targets, however this might end-up in an infinite loop if there are multiple
non-input windows stacked.
So, accept any focusable window as fallback focus target even if it's a
take-focus one (that might not reply to the request).
Added a stacking test to verify this.
Closes: https://gitlab.gnome.org/GNOME/mutter/issues/660https://gitlab.gnome.org/GNOME/mutter/merge_requests/669
