Add XML file import back and treat open and import differently

Open (or adding a file name on the command line) means that this is just
one of the files that you consider part of your dive history. So dives
don't get automagically numbered and the dive_list is not considered
"changed" just because another file was opened.

Import (or adding a file on the command line after --import) means that
you are importing the content of this file to your dive history. So if the
imported file has un-numbered dives that are newer than everything else,
those get correctly renumbered. And importing marks the dive_list as
changed.

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