First of all, derive GimpFilterOptions from GimpColorOptions, not the
other way around, which was a sick way of magically showing the right
options for various tools. That approach has failed and needed
additional hacks to do the right thing. Simply have the right class
hierarchy and call the right tool options GUI constructors and done.

Remove gimp_histogram_options_gui(), we don't need histogram scale
controls in tool options when we already have them next to the
histograms in the tool dialogs.

Also remove gimp_histogram_options_connect_view() and use a simple
g_object_bind_property() instead.