First cut at working cylinder editing dialog

This currently only does the same old things we used to do (so still no
start/end pressure or trimix support), but despite that this is already
more flexible than the old model:

 - we can now add new cylinders, rather than just edit the information of
   the first two cylinders of the dive

 - because the cylinder editing is being done in a edit dialog, it is
   now much more reasonable to use multiple lines and expand all the
   things we can edit.

But to actually make this fully fledged, we'll need to add all the other
info to the cylinder edit dialog, and probably add a confirmation dialog
for the "delete cylinder" case too.

Oh, and right now deleting a cylinder doesn't mark the dive info changed.

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