GPS location map input

[Dirk Hohndel]

Just pushed out Linus' Gtk3 readiness changes plus my change that allows
the user to not only type in GPS coordinates but also use a map widget
to pick the dive site.

There were a few very odd Gtk things going on - when I opened the map
widget from the dive info dialog (by clicking the button), the widget
would be completely unresponsive. No panning, no zooming, no
right-click, nothing.

Opening an equipent widget and immideately closing it again suddenly
made the map widget responsive. WTF?

I worked around this by doing an explicit grab in the map widget, but
that seems like a hack and just to work around the underlying issue.

If anyone can figure this out, patches welcome.

The other shortcomings (besides the uglyness of the UI) are that it may
be non-obvious to the user that it takes a right click to get a menu
item that allows you to "mark location here" - I'm sure there's a more
intuitive way to do this, but since left click is used for panning, this
was the best idea I could come up with...

Please test - I wouldn't be surprised if there are a few bugs still
hidden in this code.

/D