gimp_brushes_popup() was triggering a popup allowing to change not only
the brush, but also the "paint mode", the opacity and the spacing. As
far as I could see, this was used nowhere for the paint mode and
opacity.
As for the spacing, it looks like gfig was indeed getting this data,
except that the GimpBrushSelect was disabling the effect on this scale
by setting change_brush_spacing to FALSE when calling
gimp_brush_factory_view_new(), and even setting this to TRUE, it was not
working fine anyway. Rather than debugging this, let's simplify the API.
Such settings seems like additional paint settings which don't have to
be in the brush selection. If someone wants people to also select an
opacity, spacing or paint mode, it would be much more efficient to add
separate plug-in arguments for these.
Additionally, I fix GimpBrushFactoryView not to show the spacing scale
when change_brush_spacing was set to FALSE anyway. This is just a bogus
widget then, which can only confuse people.
