T1001450 todo markers

I deleted a lot of code+tests because the logic could be simplified a lot.

I don't know how to test the new behavior easily because it relies on UI components. Furthermore I don't think that tests are important because the behavior relies on Eclipse logic (or better is copied from Eclipse logic). I did mainly manual tests:
- Changing `Java → Compiler → Task Tags` on the fly requires no editor restart
- Comments/Scaladoc gets highlighted the same way as before