Add tag helper functions to allow us to count dives with a given tag

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Show the number of dives with each tag in the filter panel

This is a rather brute force implementation. It might be worth while
caching this information - but I'd like to do some benchmarking, first.

Sadly this code also shows that there are some bugs hidden in the
selection code when filtering is present. When a trip is selected all
dives in the trip get selected, even those that are not visible under the
current filter.

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