First step towards a context menu in the profile view

This is completely bogus as all it does is print out the corresponding
time for the spot we right-clicked on the profile. But that at least shows
that the infrastructure is working as intended...

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Manually add gas changes to a dive

Create a little widget that lists all the gases / tanks we know about and
allow the user to pick one of them.

Turns out that add_event only added events at the end of the list - but we
treat that list as chronologically sorted. So I fixed that little
mis-feature as well.

This does raise the question whether we need the inverse operation
(removing a gas change). And if there are other things that we should be
able to manually edit, now that we have the infrastructure for this neat
little context menu...

See #60 -- this doesn't address all of the issues mentioned there, but at
least deals with the 'headline' of the feature request...

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