Remove TAKEOFF from mission item fragment type list if there is more than 1 item in mission list
Allow LAND and RTL mission items to be set only in the end of mission list
Fix TAKEOFF item handling
Stop new mission items addition after RTL or LAND
Merge pull request #959 from alextel69/fix-takeoff-land-sequence

Fix TAKEOFF-LAND (RTL)  sequence

Closes #794