Showing J2000.0 coordinates in the Details dialog.

TODO:

1. Fix bugs and make sure that we are _actually_ showing J2000.0.

We _know_ this is not the case really, as per a recent discussion
on the kstars-devel Mailing List -- some stars have catalog
coordinates from a different epoch, but we assume they are J2000.0;

This leads to visible errors in matching DSS overlays, for example
(which I think I did notice at the Texas Star Party).

2. Improve the Astroinfo article on precession, to which the J2000.0
RA and Dec labels are hyperlinked. The article on precession
doesn't talk about epochs.