LibGUI: Add a GModelNotification class that views will receive.

I don't want to use GEvent here since these need to be synchronous
and mixing sync and async GEvents would be stupid.

Applications/ProcessManager/ProcessTableView.h

@@ -15,6 +15,9 @@ public:
 
     Function<void(String)> on_status_message;
 
+protected:
+    virtual void model_notification(const GModelNotification&) override;
+
 private:
     virtual void timer_event(GTimerEvent&) override;