Remember last vehicle type and commands for mission planning

Long time ago, MissionPlanner was remembering last connected vehicle type - plane for example - and did not always revert to copter for mission planning as in recent versions.

This annoying bug of not defaulting to last vehicle type disallows to offline plan a mission for the usually connected vehicle successfully without connecting to board 1st.

This also caused loading a mission for plane to have many commands as "UNKNOWN"

Tested up to latest release on 1.3.77

Start by reproducing the issue in Mission Planner 1.3.77 or the latest available release: connect or select different vehicle types, disconnect, and open offline mission planning. Verify whether the last vehicle type is retained, whether plane missions load with recognized commands instead of UNKNOWN, and whether copter is still incorrectly selected by default.

Written by the indexing model from the issue text.