New Shadow / Midtone / Highlight tool

This is another big win for PhotoDemon, as this was previously an
exclusive PhotoShop tool.  (Other free options like GIMP and Paint.NET
lack a shadow/highlight adjustment, despite it being extremely useful
under a variety of circumstances.)

The max and min values don't currently make much sense, but I haven't
made up my mind about how to better display them.  A scale of 1-100
might be better, but it needs decimal-like precision for best results.

Will revisit in the near future.