LibGUI: Add Widget::repaint() to force an immediate repaint

In most situations, Widget::update() is preferable, since that allows us
to coalesce repaints and avoid redundant work, reducing system load.

However, there are some cases where you really want a paint to happen
right away, to make sure that the user has a chance to see a short-lived
visual state.

Userland/Libraries/LibGUI/Window.h

@@ -202,6 +202,8 @@ public:
 
     Menu& add_menu(String name);
 
+    void flush_pending_paints_immediately();
+
 protected:
     Window(Core::Object* parent = nullptr);
     virtual void wm_event(WMEvent&);