Adds WebP Quality Config

[PoneyClairDeLune]

Like #3285, this PR aims at introducing a quality config into Flameshot, allowing users to customize the quality of the WebP image emitted by Flameshot.

However during the testing of this PR, I've noticed a few problems with the output emitted from the Qt5 ImageFormat plugin.

• The resulting files produced by Flameshot are inferior compared to ones emitted by cwebp with the exact same quality value.
• The Qt5 ImageFormat plugin doesn't allow saving as lossless WebP, which is a far superior lossless image format than PNG, allowing on average 27% size reduction.
• Said plugin also doesn't allow customizing effort value used to optimize the resulting image.

So perhaps, for the easier route, there should be a config option, allowing Flameshot to automatically execute a command to optimize the final image from PNG. Or maybe integrate libwebp or libvips directly?

[mmahmoudian]

@PoneyClairDeLune Thanks for the PR.

@jack9603301 What is your opinion about this question?