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/Makefile

@@ -59,6 +59,7 @@ OBJS = \
     GLazyWidget.o \
     GCommand.o \
     GUndoStack.o \
+    GDragOperation.o \
     GWindow.o
 
 LIBRARY = libgui.a