FormMain: finalize Adjustments menu for 6.4

I've decided to go with three identical options for both "Auto correct"
and "Auto enhance": color, contrast, and lighting.  These three options
will provide the same corrections/enhancements as Photoshop, but with
more accessible wording for beginners (without confusing pros, I hope).
Implement Adjustments > "Auto correct color" and "Auto correct contrast"

Auto-correct color ties into PD's existing white balance code.
Basically, each color channel is stretched across its full spectrum
(while ignoring a chunk of the histogram on either end, to prevent
outliers from ruining our analysis).  Note that this can negatively
impact an image's color balance, depending on the composition, as RGB
ratios can/will change, just as they do in Photoshop, etc.

Auto-correct contrast, on the other hand, will apply a uniform
adjustment to each channel.  An image's color balance will not be
affected, but overall contrast will.  That said, a well-exposed photo is
unlikely to see any noticeable changes from "auto contrast", as contrast
should already be well-distributed.

Lena is an excellent photo for comparing differences between these
methods.  From what I know of Photoshop's Auto methods, our results
should be pretty much identical.  (There are only so many ways to
stretch a histogram!)
FormMain: add placeholder warnings for Auto enhance menu

Unlike the "Auto correct" tools, "Auto enhance" tools exist to
noticeably punch up image attributes (instead of subtly correcting
details).  I foresee these as a way of adding some baseline amount of
life to dull photos, and simplifying the editing process for beginners.

Actual implementations TBD.