graphics optimization feature inspirated by diablo3 engine

* the rendering now consits of two panes
- the background pane (for animated stuff like the map)
- the foreground pane (for steady stuff, like UI)
each pane has it own max FPS and works idependently
this may increase graphics performance on many platforms

src/framework/ui/uiwidget.h

@@ -52,9 +52,9 @@ public:
     virtual ~UIWidget();
 
 protected:
-    virtual void draw(const Rect& visibleRect);
-    virtual void drawSelf();
-    virtual void drawChildren(const Rect& visibleRect);
+    virtual void draw(const Rect& visibleRect, bool foregroundPane);
+    virtual void drawSelf(bool foregroundPane);
+    virtual void drawChildren(const Rect& visibleRect, bool foregroundPane);
 
     friend class UIManager;