Invert compression scale which appear during file saving, to be more intuitive.
After applying this patch, on left side there will be low JPEG/PDF/WebP compression and high quality, on right side high compression/low quality.
The "Compression" label could be reused also for PNG lossless compression as it is also gives compression levels, without quality loss. For PNG format, 0 is minimal compression, and fast decoding, 9 means high compression ratio and slow decoding.
Fixes #156 (closed)