Change GimpHistogram to take a "gboolean linear" parameter and always
honor that parameter, so both kinds of histograms can now be created
for all drawables.

Add a horrible "Linear" toggle to the histogram dockable which always
defaults to the active layer's actual pixel format, but can be
switched at any time. This UI is ugly and needs to change.

On the PDB, default to gamma-corrected if the plug-in is unaware of
higher precision, and to the drawable's native pixel format otherwise.

Other places using histograms (e.g. levels, curves) are unchanged.