window: Return focusable ability looking at properties only

As per commit 43633d6b, we mark an unmanaging window as not focusable, while
this is true, it might cause not resetting the current focused window when
unmanaging it causing a crash.

Also this wouldn't allow to check if a window can be focused when unmanaging it,
so let's revert the previous behavior.

Fixes: https://gitlab.gnome.org/GNOME/mutter/issues/462

src/core/stack.c

@@ -1224,6 +1224,9 @@ get_default_focus_window (MetaStack     *stack,
       if (window->unmaps_pending > 0)
         continue;
 
+      if (window->unmanaging)
+        continue;
+
       if (!meta_window_is_focusable (window))
         continue;