Plumb menu item activation events from WindowServer to clients.

GMenu now has an "on_item_activation" callback that fires whenever one
of its items are activated. The menu item identifier is used to distinguish
between items.

Use this to implement font switching in Terminal. :^)

Applications/Terminal/Terminal.h

@@ -19,6 +19,7 @@ public:
     void on_char(byte);
 
     void flush_dirty_lines();
+    void force_repaint();
 
 private:
     virtual void event(GEvent&) override;