Picture handling: parse and convert old style picture events

Speacial case handling for event type '123' to instead add a picture to
the picture_list of the dive.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Picture handling: switch to stronger typed offset

Also change the on file XML to be even easier to read by making it a
duration as well (which gets us '32:34 min' instead of un-typed seconds).
This is backwards compatible, it will happily read what was written with
the previous commit).

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