minor Rally Point glitches

- when you add a rally point it pops up a dialog asking for the altitude. If you click cancel you still get one. It should either cancel the creation of that point, or we should lose the 'cancel' button from that dialog
- 'clear rally points' clears them from both MP and the vehicle, which isn't symmetrical with the other rally point command behaviour. When you add or remove one it only does it within MP, not the vehicle, so clearing them should work the same way
- if you connect and load rally points from the vehicle then disconnect and hit 'clear rally points' it throws an exception (assuming from the above behaviour, so if that changes this bug goes away)

This is all in 1.2.99

Reproduce the rally point altitude-cancel, clear, and disconnect flows in Mission Planner 1.2.99, comparing Mission Planner and vehicle state. Trace the rally point command entry points for adding, removing, loading, and clearing points; done means Cancel does not create a point, Clear matches the other command scopes, and clearing after disconnect does not throw.

Written by the indexing model from the issue text.