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>
Add Gtk dialog to select the dives tags for which dives are shown

This literally just creates the dialog and does not hook things up with
the dive list.

The idea is to abstract out the idea behind the invalid dives to allow the
user to select / deselect all kinds of dives and then do statistics on
the selected ones.

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

This only allows for a logical "AND" - so only dives that all of the
selected tags are displayed.

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