Remove old auto-enhance filters

As described in an earlier commit, these will soon be resurrected as
"auto-" functions, like "auto-levels"
Minor Vibrance adjustments

-200% results in funky images, so the slider now goes from -100% to
+200%
Improve comments
New universal selection tool dialog

Selection tools don't provide a live preview, so it's easy to create a
general-purpose dialog that can be reused for any menu-based selection
modifier (grow, shrink, feather, etc).