Remove obsolete DESTROY method.

App::SourcePlot::Source::DESTROY() would never do anything
because the class never has a MW attribute.
Add Galactic and Az/El coordinate choices.

The "New" source button now allows these coordinate
types to be selected.
Improve handling of Galactic coordinates.

Return Galactic coordinates in decimal degrees.

Detect sexagesimal / degree coordinates and specify
the unit to Astro::Coords to prevent it guessing
between degrees and radians.