Create 2d mode

[johnarban]

This begins work on #312 - creating a basic 2D mode switch, and adds a temporary box for changing the 2D sky background (default is Mellinger). When switching, it remembers the view the user had previously and adds a view reset button.

TODOs:

• We may want to remove the best fit in 2D view, though maybe not. The view is a bit confusing (it's a projection effect)
  . I am not sure that the model is actually displaying properly in the 2D view. If while in 3D i zoom all the way into the solar system and lookout, is see
  

[patudom]

Closing this because it is now duplicated in #319 and the last commit starts #320.