[droppable] Add the :enabled property
[droppable] Add the :enabled property

We kind of assumed that the Droppable interface, like the Draggable
one, has an :enabled property. Now we need to actually install the
property inside the interface.

pwithnall/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx