Improve time marker handling and add printing of some time labels

We now draw time markers at most every 5 min, but no more than 12 markers.
For convenience we do 5, 10, 15 or 30 min intervals.
This allows for 6h dives - enough (I hope) for even the craziest divers -
but just in case, for those 8h depth-record-breaking dives, we double the
interval if this still doesn't get us to 12 or fewer time markers.

We label the first and then every other time marker with the minute text.

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

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