Add new gases to the combo box in dive plan entry

Once again Gtk does everything it can to make our lives miserable. It
requires major hackery to be able to add new gases to the drop down lists
"on the fly". Right now this only works if you edit the gas and then use
Tab to move to the next field.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Make the dive plan input much more compact

Have the frames just for the top row and get rid of all of the extra space
in the layout.

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