...and present linear RGB Histograms

This is step one: implement the feature at all (without new defaults
or proper GUI, cough).

Add boolean "linear" properties to GimpOperationPointFilter,
GimpCurvesConfig and GimpLevelsConfig.

In the filter, simply set the input/output formats to linear in
prepare().

In the curves and levels tools, add "Linear" toggles from hell,
like in the histogram dockable, and make sure things work right
wrt changing and resetting the property, switching from levels
to curves, and picking colors.

The result currently changes when switching a non-nop curves/levels
between perceptual and linear, because adjusting the parameters
between the spaces is not implemented yet.