Distinguish between "collapse" and "collapse all"

Some users might want to really see nothing but trips, others might want
to be able to keep the trip with the selected dive open.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Only show expand/collapse context menu when in tree mode

The options make no sense when in list view mode.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Implement the context menu a different way

It seems that this is the way this is supposed to be done - instead of
manually looking at what kind of click we get, Qt decides when to create
a context menu for us - this way things like the Windows Menu button
will work automagically.

As an example I also implemented the "remove dive from trip"
functionality, which exposes some other bugs (like the fact that the
dive that isn't part of a trip ends up being sorted at the very end of
the dive list).

This commit contains a "testSlot" implementation to remind me how to
figure out which dive / trip we are on.

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