Highlight notes under playback cursor

[bluebear94]

Resolves: #11313

Re-adds the MuseScore 3 feature where notes under the playback cursor are highlighted during playback.

This PR is a draft because the implementation is extremely hacky. I don’t think PlaybackCursor is the right place to implement this. MuseScore 3 implements this feature in Seq::heartbeatTimeout, but as the playback engine in MS4 is completely different, I have no idea where this is best implemented.

Also, arpeggios and grace notes are not highlighted properly.

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