- Moved 'exclusiveGroup' property from ButtonBehaviour to BasicButton.
- Renamed registerCheckable() to bindCheckable(), unregisterCheckable()
  to unbindCheckable().
- Improved and updated documentation.

Change-Id: I74cdea37cb8fcdf5cce22f58981cfb3a7dafaf8f
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>