When manually adding a dive allow user to set avg depth as well

So far we only allowed setting the max depth.

This changes the layout of the entry widget and makes our helper function
create_date_time_widget return the hbox in which it positions the time
entry. I plan to reuse this later when allowing to edit the duration as
well as the start date and time under certain circumstances.

This is a small part of a feature request; see #75

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Added back support for versions of GTK pre 2.24

Some functions in combo_box_with_model_and_entry() are only
available to GTK 2.24 and newer. This patch adds only one #if branch,
but probably adds support to a number of outdated distributions,
such as Debian 6.0.4 and Ubuntu 10.04.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>