Handle double clicks on globe during dive edit mode

When we are editing or adding a dive, the globe widget needs to act
differently. Instead of directly changing the lat/lon of selected dives,
it needs populate the coordinate text field as if this information was
entered by the user (effectively all it is is a way to more conveniently
enter coordinates).

As a side effect, this also allows us to change the location once it has
been added (you just need to go into dive edit mode to do so).

There is one weird issue that occasionally (and I can't quite reproduce
this) I get lat/lon very close to 0/0 (as if the globe widget was centered
on 0/0 and not on whatever it is actually showing). That still needs to be
addressed.

Fixes #239
Fixes #131

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Immediately show new locations on globe widget

When modifying or adding a dive location on the globe widget while in dive
edit or dive add mode, we now show the modified / new location right away
on the globe (assuming a location name has been given). This makes it much
easier to manually reposition a dive location.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Immediately update the dive flag label when editing location text

This seems like a much nicer user experience. Editing a location's name is
immediately reflected on the globe label, the moment it happens.

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