Fixed Infinite map zoom

We set the PlanetariumCamera to the selected vessel and after this we removed this new "entry" from the PlanetariumCamera.fetch.targets list. (FocusFragment.cs:45) If you switched back to the map view the PlanetariumCamera is set to a vessel that doesn't exists on the PlanetariumCamera.fetch.targets list anymore.

This will fix #188
Not sure about this one. Wouldn't it be easier for the user if the active vessel does show up on the list? They might find it more convenient than using backspace to get back.