Add "enable" button for manual pressure setting

This grays out the pressure settings in the cylinder editing widget if
the pressure data has been taken from the samples.  You can still
manually override the data, but you now need to enable that manual
override explicitly.

This makes the semantics of editing start/end pressures of dives with
pressure sample data a bit more intuitive, I think.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>