Pick GPS coordinates of dive location via map widget

I have some concerns about the way this is implemented - especially the
use of gtk_grab_add to make the map widget work has me worried. But it
seems to work and survived some test cases that I threw at it.

The GtkButton with the Pixmap looks a little off on my screen, but this
way it was easy to implement. Feel free to come up with a better design.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Clean up gtk combo box handling

This cleans up our handling of combo boxes and all the duplicated
completion logic, and simplifies the code.

In particular, we get rid of the deprecated GtkComboBoxEntry.  While it
made some things easier, it made other things harder.  Just using
GtkComboBox and setting that up correctly ends up being simpler, and
also makes the logic work with gtk-3.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Prepare to compile with gtk-3

To actually compile with gtk-3 you currently need to disable the use of
OSM (compatible with gtk2 only for now).  And obviously figure out the
headers and libraries by using pkg-config on gtk-3 instead of gtk-2.
But now it should be only a Makefile change.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Merge branch 'gps-map-input'

Add ability to pick GPS coordinates of dive locations from a map widget.