Bug 721067 - Opacity slider broken in "Edit/Fade [plugin]..." dialog

gimp_prop_opacity_entry_new(): need to pass FALSE not TRUE to
gimp_prop_scale_entry_new()'s limit_scale parameter, it has the
inverse meaning than the same boolean in gimp_scale_entry_new().

This is perhaps a bug, but we can't change that now; instead, fix the
argument names and docs of gimp_prop_scale_entry_new() to point out
the difference.