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);

SharedGraphics/Rect.h

@@ -99,6 +99,13 @@ public:
         return rect;
     }
 
+    Rect translated(const Point& delta) const
+    {
+        Rect rect = *this;
+        rect.move_by(delta);
+        return rect;
+    }
+
     bool contains(int x, int y) const
     {
         return x >= m_location.x() && x <= right() && y >= m_location.y() && y <= bottom();