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.

SharedGraphics/Painter.h

@@ -52,6 +52,8 @@ public:
     void translate(int dx, int dy) { m_translation.move_by(dx, dy); }
     void translate(const Point& delta) { m_translation.move_by(delta); }
 
+    GraphicsBitmap* target() { return m_target.ptr(); }
+
 private:
     void set_pixel_with_draw_op(dword& pixel, const Color&);
     void fill_rect_with_draw_op(const Rect&, Color);