[gtkmm] Drag And Drop

["Ohrnberger, Erik" <erik_ohrnberger dme net>]

	Given that a drag and drop operation has been successful, and that
we are on the drag_drop_data_received method of the target, is there anyway
to find out which combination of source and target strings matched up to
make this an allowed drag and drop operation?

This would help in the data_received method to determine the appropriate
actions to take (based on what was dropped).

Thanks,
	Erik.