libdivecomputer integration: add a progress bar

libdivecomputer integration: add a progress bar

Instead of writing out the progress events, use them to update a real
progress bar.

Also, we need to handle gtk events while busy with the dive computer
reading.  That should *probably* be done with a threading model, because
libdivecomputer does seem to have some timing sensitivity - I'm getting
"failure to read memory block" if I make that loop do the standard

	while (gtk_events_pending())
		  gtk_main_iteration();

thing.  Besides, even if we did do that loop, it would still cause
problems when the libdivecomputer code is stuck reading a serial line
that doesn't respond or whatever.

But for now this ugly hack is "good enough" to get further.

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