LibGUI: Fix widgets not being occluded

Widget::is_visible_for_timer_purposes needs to also consult with the
base implementation, which ultimately checks the owning Window's
visibility and occlusion state. Widget::is_visible merely determins
whether a widget should be visible or not, regardless of the window's
state.

Fixes #8825

Userland/Libraries/LibGUI/Widget.cpp

@@ -1135,7 +1135,7 @@ bool Widget::has_pending_drop() const
 
 bool Widget::is_visible_for_timer_purposes() const
 {
-    return is_visible();
+    return is_visible() && Object::is_visible_for_timer_purposes();
 }
 
 }