Add simple debug function to planner

This allows you to dump the diveplan structure

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix two bugs with the gas entry code in the dive planner

First, I forgot to pass in the idx into the gas callback function - this
way for the dynamically created dives we always used air for anything but
the first segment.

Second, when selecting a gas from the drop down (with the mouse or by
typing), the GtkEntry doesn't receive that text and therfore we never
picked up those gases.

We now also track the 'changed' event for the GtkComboBox, but never add
the text we get their to the completions (as by definition they are
already there).

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