Send a cancelled() event if the dnd data source is not accepted
Currently, Firefox will stuck in "dnd" mode if you try to drag any of its tabs. The main reason for that is that kwin doesn't send the wl_data_source.cancelled event if the pointer button has been released and there is no target surface or if the data source hasn't been accepted by anyone.
CCBUG: 427528
Edited by Vlad Zahorodnii