libdivecomputer log and dump

[Dirk Hohndel]

I merged some code from Jef to enable saving either the libdivecomputer
log or a complete dump from within Subsurface. I hooked this up from the
dive computer download dialog, but ran out of time to test this much.

Please poke at it and see if it works and does what we need. The reason
to include this in Subsurface is simple. In 90% of all cases where
people have issues with dive computer download, the very first thing we
need are one or both of these files. And so far we always have to ask
people to download the libdivecomputer applications - it makes much more
sense to simply offer this from within Subsurface.

Happy Holidays to those of you celebrating :-)

/D