Uemis downloader: show the timestamp for dives while downloading

If we can easily decode the start time of the dive that is currently being
parsed (i.e., if the information doesn't happen to straddle a block
boundary) then show that in the dialog's progress bar.
Make the explanatory text before it shorter so there's enough space.

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