Improve: Allow for custom descent/ascent rates while building a profile (based on time)

[pabdakine]

Would like a gradual descent/ascent where the rate is calculated from the depth difference (from previous segment) decided by the specified time. A common profile will be a straight drop, a gradual descent along a sloping bottom, then a level segment.

Interface-wise, I suggest an icon pointing down/up (after depth is entered) that can be tapped to point diagonally up/right or down/right.

[Rolf-Smit]

@pabdakine I have this on my own internal backlog, the whole profile builder could use some more advanced features indeed, this being one of them.