LibGUI+WindowServer: Start fleshing out drag&drop functionality

This patch enables basic drag&drop between applications.
You initiate a drag by creating a GDragOperation object and calling
exec() on it. This creates a nested event loop in the calling program
that only returns once the drag operation has ended.

On the receiving side, you get a call to GWidget::drop_event() with
a GDropEvent containing information about the dropped data.

The only data passed right now is a piece of text that's also used
to visually indicate that a drag is happening (by showing the text in
a little box that follows the mouse cursor around.)

There are things to fix here, but we're off to a nice start. :^)

Libraries/LibGUI/GWidget.cpp

@@ -156,6 +156,8 @@ void GWidget::event(CEvent& event)
         return handle_mouseup_event(static_cast<GMouseEvent&>(event));
     case GEvent::MouseWheel:
         return mousewheel_event(static_cast<GMouseEvent&>(event));
+    case GEvent::Drop:
+        return drop_event(static_cast<GDropEvent&>(event));
     case GEvent::Enter:
         return handle_enter_event(event);
     case GEvent::Leave:
@@ -350,6 +352,11 @@ void GWidget::change_event(GEvent&)
 {
 }
 
+void GWidget::drop_event(GDropEvent& event)
+{
+    dbg() << class_name() << "{" << this << "} DROP  position: " << event.position() << ", text: '" << event.text() << "'";
+}
+
 void GWidget::update()
 {
     if (rect().is_empty())
@@ -686,4 +693,3 @@ Vector<GWidget*> GWidget::child_widgets() const
     }
     return widgets;
 }
-