Implement quality setting for WebP export

Since WebP is the only format handled by QImageIO plugin that
uses the quality option (or any meaningful options really),
there are no generic QImageWriter capability checks for now.

A quality of 100 makes the WebP plugin bundled with Qt save in the
lossless format. This is nice because in many cases it compresses
significantly better than PNG, including alternative implementations
that put additional effort into compressing PNGs better then libPNG.