Move creation of dive and dive trip date string into helper functions

This allows this code to easily be shared by Gtk and Qt UI.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Use helper function to get dive date string

Correct order of arguments in the DiveItem constructor call.

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