Redo dive editing

Redo dive editing

This commit addresses two issues:

We now can add / edit / delete equipment from the edit dive dialog

We now can edit multiple dives at once

The latter feature has some interesting design constraints:
It picks the 'selected_dive' as the one to start the edit from - so if
this dive already has some information filled in, that information needs
to be overwritten before it is stored in all of the dives. Similarly, only
changes to the cylinders or weightsystems are recorded. Also, the notes
field is not editable in the multi dive edit mode (as that didn't seem
useful).

The workflow seems to work best if using the multi-edit right after
importing new dives from a dive computer. The user then can select all the
new dives and only needs to edit things like location, divemaster, buddy,
weights, etc. once.

This commit will create some obvious conflicts with the commit that adds
exposure protection tracking. It was implemented on top of the tree_view
changes as it reuses some of the infrastructure for tracking the selected
dives.

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