Make the divemaster/buddy/location text entries be combo box entries

The text entries have completions, but if you want to see the full list
of possibilities, I'm not seeing how to do that without turning the
GtkEntry into a GtkComboBoxEntry.

The list of people/locations are not sorted, though, which makes the
full list less than readable.  Will have to do that too.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>