include new definitions and updated versions of existing definitions
"default/classic" and "modern" has been renamed to "Parchment" and "Celes" to avoid confusion
It has been a while since the last optipng run (it was in 2015),
and many non-optimized PNG files have accumulated in the repository.
288 of 1666 files optimized, 258992 bytes reduced to 156258 bytes; -102734 bytes, -39.66%
To avoid unnecessarily increasing the size of Git repository for minor
size reductions, we skip images that weren't reduced by at least 15%.
