Clear dangling drag-focus resource on client disconnect

DataDevice::m_dragFocusResource pointed at whichever client's
wl_data_device currently held drag focus, with no destroy-tracking. If
that client disconnected mid-drag, the pointer was left dangling, and
the next pointer motion during the still-active drag would dereference
it in setDragFocus().

Add a data_device_destroy_resource() override to clear
m_dragFocusResource/m_dragFocus when the resource they point at is
actually destroyed.

Pick-to: 6.8 6.11 6.12
Task-number: QTBUG-148611
Change-Id: Ib6c7136bc8ae3fe56dd9b340fd1d0d26acf0e7bf
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>