Addition / change to the user manual

- give an introduction to the dive list and explain some behaviour of dive
  list to new users
- a small clarification regarding the used Buhlmann-algorithm

Signed-off-by: Thomas Maisl <exp-122004@maisl.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Disable OK button in download dialog when downloading

The button was active while downloading, and that was
confusing.

Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>