Mark locations that have GPS location data attached

This is rather lame - we simply turn the location text into italics for those
dives where we have GPS location data. Underlining might be more natural, but
Gtk plays games with the underline attribute if the mouse hovers over text.

Ideally I would have prefered a little GPS logo next to the location text - but
I couldn't figure out how to do that without writing my own cell renderer which
seemed total overkill.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>