[WIP] Filter rewrite

This is a work in progress, discussion/feedback goes here.

This is in LATEST builds.
https://github.com/MayhemYDG/4chan-x/wiki/4chan-X-LATEST

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The goal of this rewrite is to make it easier to write filters, to fix some pain points, and to add functionality.
See CHANGELOG.md for what changed.

TODO list:
- [x] Filter exceptions. #792
- [x] Optional desktop notifications when highlighting.
- [ ] Fix labels not showing the correct content(s) that matched the filter.
- [ ] Add ability to test what posts a filter matches.
- [ ] Add ability to apply/unapply/update filters without refreshing, maybe? #653
- [ ] Improve the UI.
- [ ] Filters should be reorderable.
- [ ] Disabled filters need some emphasis.
- [ ] Write full guide.