Port Terminal to LibGUI.

To facilitate listening for action on arbitrary file descriptors,
I've added a GNotifier class. It's quite simple but very useful:

GNotifier notifier(fd, GNotifier::Read);
notifier.on_ready_to_read = [this] (GNotifier& fd) {
    // read from fd or whatever else you like :^)
};

The callback will get invoked by GEventLoop when select() says we
have something to read on the fd.

LibGUI/GWidget.h

@@ -58,7 +58,7 @@ public:
     Size size() const { return m_relative_rect.size(); }
 
     void update();
-    void repaint(const Rect&);
+    void update(const Rect&);
 
     virtual bool accepts_focus() const { return false; }
 
@@ -136,6 +136,7 @@ private:
     SizePolicy m_vertical_size_policy { SizePolicy::Fill };
     Size m_preferred_size;
 
-    bool m_has_pending_paint_event { false };
+    Vector<Rect> m_pending_paint_event_rects;
+
     bool m_fill_with_background_color { true };
 };