MissionList: Further reduce the chance of unintentionally deleting waypoints - dragRemove

Added 'dragRemove' remove mode to DragSortListView - configured mission list to use drag vs fling remove.

Row must be dragged past row.width / drag_width_divisor to be removed.
See: fragment_mission.xml - drag_width_divisor set to 4.

Related to https://github.com/arthurbenemann/droidplanner/pull/312