PixelPaint: Apply filters on another thread

[kleinesfilmroellchen]

The main advantage of this change is that heavy-weight filters do not lock up the GUI anymore.

This first cut has several flaws:

• We do not account for modification of the referenced images while the filter is running. Depending on the exact filter behavior this might have all sorts of weird effects. A simple fix would be to show a progress dialog to the user, preventing them from performing other modifications in the meantime.
• We do not use the image processor for previews. Preview behavior has a couple of other considerations that are intentionally not addressed in this commit or pull request.

CC @Tobyase, this is the first PR that addresses points laid out in #14843.

[djwisdom]

More ❤️ to PixelPaint, let's go! :^)

[TobyAsE]

All this Threading looks like magic to me, I'm not qualified to review this. But reading the code and understanding basic concepts it all looks sensible.

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!

[TobyAsE]

@kleinesfilmroellchen can this be merged or are there any unresolved issues left?

[kleinesfilmroellchen]

It can be merged but will need some reworking later. It's good for a first cut though.