Rough "Add new dive" infrastructure in the divelist

Do a right-click to get a menu with the "Add dive" entry.  Should do
delete too, but that's for later.

What's also apparently for later is to make this *useful*.  It's the
butt-ugliest time entry field ever, and there's no way to set depth for
the dive either.  So this is more of a RFC than anything truly useful.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>