Be willing to unfocus the grab window when we are unmanaging it

When we are unmanaging the grab window, we /need/ to unfocus it,
so we shouldn't bail out early from meta_window_focus().

https://bugzilla.gnome.org/show_bug.cgi?id=711618
display: Don't leave focus on a window we are unmanaging when sending WM_TAKE_FOCUS

When we move focus elsewhere when unmanaging a window, we *need* to move
the focus, so if the target is globally active, move the focus to the
no-focus-window in anticipation that the focus will normally get moved
to the right window when the target window responds to WM_TAKE_FOCUS.

If the window doesn't respond to WM_TAKE_FOCUS, then focus will be left
on the no-focus-window, but there's no way to distinguish whether the
app will respond or not.

https://bugzilla.gnome.org/show_bug.cgi?id=711618