Clean up plugin readme and license files in /Plugins folder

Previously, it wasn't always clear which license and README files went
with which plugins.
Plugins: Replace pngnq-s9 with pngquant

Fixes #125.

Removes primary blocker for #118.

Warning: large commit!

Wholesale replacement of plugins is not a pleasant task, but sometimes
it needs to be done.  As pngnq-s9 has been abandoned by its creators,
and pngquant has seen huge improvements over the past two years, it was
time for PhotoDemon to make the switch.

New custom features for pngquant are available via Tools -> Plugin
Manager, and as before, its use during PNG saving is fully automatic,
depending on the needs of the saved PNG file.  Performance is
much-improved over pngnq-s9, and improvements to pngquant has greatly
closed the quality gap, so this should be a positive change.

To test the new plugin, enable manual color depth selection from the
Tools > Options > Saving section, then manually save any 32bpp image as
an 8bpp PNG.

Dedicated export options from the File menu and Batch Convert tool
(which would make access to pngquant's features much easier) keep
getting pushed back in favor of more pressing work, but this commit
removes one of my primary blockers for working on such features, so
maybe they'll show up sooner rather than later...?