Allow drag-and-drop from the Month View

Allow drags from the Month View to, e.g., a KMail composer.
The MonthScene uses an invalid QDate to announce a drag outside of the
calendar grid, and the IncidenceMonthItem cancels the move-in-progress
and creates a QDrag.

CCBUG: 182867
3 jobs for !75 with drag-from-month in 3 minutes and 13 seconds (queued for 3 seconds)
merge request