WindowServer+LibGUI: Add data_type and data fields to drag operations

These fields are intended to carry the real meat of a drag operation,
and the "text" is just for what we show on screen (alongside the cursor
during the actual drag.)

The data field is just a String for now, but in the future we should
make it something more flexible.

Libraries/LibGUI/GModel.h

@@ -36,6 +36,7 @@ public:
         BackgroundColor,
         Icon,
         Font,
+        DragData,
     };
 
     virtual ~GModel();