DnD fixes
Fixes a possible crash if a DnD wl_data_source is destroyed after the grab is over, but before the drag dest acknowledged the drop, and other small fixes spotted along the way.
Closes: #591 (closed)
Fixes a possible crash if a DnD wl_data_source is destroyed after the grab is over, but before the drag dest acknowledged the drop, and other small fixes spotted along the way.
Closes: #591 (closed)