Small issue when entering a dive plan

[Dirk Hohndel]

I haven't spent any time looking into this, but I noticed that I cannot
change the duration of any segment of a dive plan in the data table. You
can change the depth, but not the duration.

To reproduce:
Start Subsurface
Close your data file (Ctrl-W)
Start planning a dive (Ctrl-L)
In the table widget under "Dive Planner Points" try and change the value
in the second column, second row (currently a duration of 39 minutes).

As soon as you try to type a number, it jumps to a matching row (so one
where the duration starts with that number - say you enter '1' in order to
change the duration to "10", you'll end up in the first row), but you
cannot modify any of the durations.

/D