Turn off Qt including the C++ Standard Library headers

We don't use any of the Standard Library features or classes in
Subsurface, except for three uses of std::sort, for which I added the
necessary #include.

It's always a good idea to include directly the headers you want,
without relying on indirect inclusion

On my machine, the build time dropped from 16.96s to 13.38s or a 12%
improvement.

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Lookup gas via the plot_data, not the event

When straight decoding the event, it failed to handle older
SAMPLE_EVENT_GASCHANGE(11), the ones without he-part correctly. This was
shown clearly when it printed the 21/35 in dives/test20.xml as air.

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>