Dive d/l UI: redo the states, the flow, the buttons

This is bigger and more invasive then I wanted, but it's hard to break it
down into smaller pieces. Here's what it does:

The former "Download" button becomes the "Download", "Cancel download" and
"Retry" button. So this button controls your interaction with the dive
computer.

The other two buttons are now purely "OK" and "Cancel" for the dialog.
"Cancel" discards what happened (much easier now that we download into a
different table), and "OK" adds the dives that were selected in our
selection UI (by default all downloaded dives) to the real dive_table.

And while redoing all this, I also redid some of the state machine
underlying the dialog. The biggest change that the user will see is that
partial downloads (after canceling or after an error) will still offer the
dives that were completely downloaded up to that point in the selection
menu.

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

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