Show the gas with the pressure diagram

This is a feature that had been requested a few times in the past and when
debugging my "show only used gases" commit I realized that this would have
been extremely useful to have...

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Merge branch 'showGas'
Much better solution to avoid the gtk+ style on Linux

Thiago showed me how to find out which style is in use and if we see a
user is running gtk+ we simply switch Subsurface to Oxygen (using the
old plastique in the previous patch was a result of reading an old Qt
book, I guess).

Solved-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>