Refactor the QPA dnd interface.

Refactor the QPA dnd interface.

- Give QPlatformDrag a synchronous drag() function returning
  the Qt::DropAction
- Move the base functionality for asynchronous event handling
  to the platformsupport library as QBasicDrag (extendable base class
  handling drag icon and providing new virtuals)  and QSimpleDrag
  (sample implementation for drag within the Qt application).
- Change the Windows implementation accordingly.
- Change XCB to be based on QBasicDrag.
- Clean up QDragManager.

Change-Id: I654f76f0e55a385ba189bd74f3ceaded6a8fe318
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>