Calculate minimum and maximum heartrate

And setup the axis accordingly.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
New profile: better positioning for the heartrate labels

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
New profile: create new class for DiveHeartrateItem

This allows us to give it a different color (red) and make it a smaller
size.

While implementing this I also fixed the size of the temperature text in
the new profile.

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