Indirect phase angle

[Gnurfos]

The "phase angle" is displayed in "rendez-vous" information when a target is selected.
Currently only 2 cases seem to be handled correctly:

• I am orbiting the same body as my target
• the target is orbiting the same body as my "reference body"

I think the algorithm could be a bit more generic, and search the closest common ancestor between me and my target, and take its 2 descendants' orbits.

Example: I am orbiting Laythe and want to get back to Kerbin. Currently I think KER is displaying a meaningless angle between Laythe and Kerbin.

Sun > Kerbin(target)
Sun > Jool > Laythe > Ship

The common ancestor is the Sun. The 2 descendants are Kerbin and Jool. The phase angle displayed would be that between Kerbin and Jool.

I think the calculation is easy (sorry I'm not familiar with the language and don't have windows so I cannot do it); the challenge would be to show to the user somewhere what bodies are used. However, the current situation is already quite cryptic, so not showing anything would not be a regression.