Include unused tanks in merges of multiple logs into a single dive if
the 'Show unused cylinders' preference is enabled.
Also rename the preference (in code) to `include_unused_tanks` to
reflect the fact that it is already used in more places than just the
display (exporting, cloning dives).
Simplified the cylinder model to make forced inclusion of unused tanks
dependent on use of the model in planner.
Leaving the persisted name of the preference as `display_unused_tanks`
to avoid resetting this for all users - is there a good way to migrate
preference names?
Signed-off-by: Michael Keller <github@ike.ch>
