Update Scaladoc with task tag highlighting

It turned out that he new introduced `ScalaCommentScanner` can
be used to simplify `ScaladocTokenScanner` a lot.

The test suite has changed in so far that it relies now on the
output of the scanner directly instead of the separation of its
internal tokenizing logic.

Some tests had to be adjusted too. The old behavior would
unnecessarily complicate the scanner logic without improving
syntax highlighting much. Affected are only corner cases where
the new behavior doesn't make comprehensive differences.