mission plugin: Clear old mission items #348
mission plugin: Clear old mission items #348

Before importing mission items from a QGroundControl plan, we've
to clear old ones; otherwise they would reside along with new mission
items, which is not expected. This commit clears them before importing
mission items.

dronecore/DroneCore,dronecore/DroneCore,dronecore/DroneCore,dronecore/DroneCore