rally: Allow rally point's altitude to be updated without deleting.

Deleting and recreating a rally point is unnecessary when all one needs to do is change the altitude(s).  Comes up from time to time when auto landing.

Also drew a line around the rally icon.  Makes it easier to tell where the icon's center is.

Note that this pull request is dependent upon another with the same branch name in mavlink:

https://github.com/mavlink/mavlink/pull/218