Only offer to show dive on map if we have location information

This adds the "Show in map" menu entry to the divelist only if we
actually have a location to show.

Of course, having some way to visually see whether we have a GPS
location even before we show the menu would probably be good. Maybe a
marker in the "location" string or something. But in the meanwhile, at
least we don't have that menu entry if we have nothing to show.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Update dive flag image.

The previous flag didn't work on Windows somehow.  I guess Photoshop
added some headers that GdkPixbuf didn't like.  I've reconverted it
with ImageMagick, which made the file less than half the size. Bonus!

Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>