Add gas consumption statistic for selected dives

We already did a list of gases and volume consumed for the selected dive
on the Dive Info tab, but did not provide that same data on the Stats tab
for all the selected dives.

I arbitrary limited this to eight gases (as the list can get quite long
when you select a lot of dives). The gases are sorted by volume consumed.

Fixes #535

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

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