DnD/X11: improve callbacks invocations.
DnD/X11: improve callbacks invocations.

There is no reason why drop targets callbacks registered for a specific
type would be invoked when a not supported data is dragged.
This patch fixes it by comparing the data type and the callback type of
the drop target. Only the callbacks supporting the data type are
invoked.

FlorentRevest/Elementary,tasn/elementary,rvandegrift/elementary,rvandegrift/elementary,tasn/elementary,tasn/elementary,tasn/elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,rvandegrift/elementary,rvandegrift/elementary,FlorentRevest/Elementary,tasn/elementary