Default Tk::AstroCatalog::getSource to RJ

The search box now appears with RJ "Epoc" already selected.
Implement Tk::AstroCatalog::getSource by name

The search interface previously called a method search_catalog
which doesn't exist.
Implement Tk::AstroCatalog::getSource by position

If RA and Dec are both specified we can use
Astro::Catalog::filter_by_distance, otherwise we
have to use a custom callback to filter by RA
or Dec alone.

Added a distance menu rather than attempting to
re-implement the original SourcePlot part-based
matching. I think this makes it more obvious
what's going on.