Add support for different coordinate frames

We should add support in the viewer state and viewer state widget for selecting the coordinate frame in which the coordinates are defined. This could include for example the ability to choose a solar system body, and the attributes for longitude/latitude/altitude, or the ability to choose a celestial coordinate frame (ICRS, FK5, Galactic). We could then always convert this to XYZ in this plugin and use the correct reference frames in OpenSpace.