Convert JPEG dialog to PD's dedicate preview user control

This allows the user to pan around the image at 100% if desired, or to
toggle between original and preview versions of the image.

Form layout has also been standardized to match all other PD dialogs.
New feature: automatic JPEG quality determination

...using a user-friendly value of "how much can we destroy the image?"

This is a pretty kick-ass feature, if I do say so myself.  When enabled,
PD will intelligently search JPEG compression values until one is found
with an acceptable RMSD (see http://en.wikipedia.org/wiki/RMSD)
corresponding to the user's quality allowance (e.g. "minor differences
allowed", or "no noticeable differences allowed").  See
http://en.wikipedia.org/wiki/Color_difference for details on how these
values are set.

At present, a maximum quality L*a*b* transform is used, which is
extremely slow, but offers ideal results based on human color
perception.  I will shortly be adding an RGB-only comparison that
operates much more quickly, but with less accuracy.

The differences between calculated quality values is staggering.  With a
setting of "barely noticeable differences", some photos can drop below a
quality of 50 (!!) while others may need high 90's to be okay.  It's one
thing to know that JPEG compression is inherently unpredictable, but
another entirely to see the results on specific images...

(NOTE: this feature is disabled for batch conversions at present.)
Performance improvements to automatic JPEG quality analysis

The user can choose to apply the quality tests in RGB space instead of
CIELAB; this hugely improves performance, and for most images, shouldn't
make too big of a difference in the final result.

Also, added tooltips for the new additions to the JPEG dialog.