Tissue saturation plot a la Sherwater Pretel

This adds a toolbox icon to turn on a tissue plot inspired by the bar
graph of the Sherwater Petrel,

It shows the inert gas partial pressures for individual compartments. If
they are below the ambient pressure (grey line) they are shown in units of
the ambient pressure, if they are above, the excess is shown as a
percentage of the allowed overpressure for plain Buehlmann. So it has the
same units as a gradient factor. Thus also the a gradient factor line (for
the current depth) is shown.

The different tissues get different colors, greener for the faster ones and bluer
for the slower ones.

Positioning and on/off icon action still need some tender loving care.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

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