Fix PianoRoll playhead not moving during Record-Play

[regulus79]

This pull request fixes #7351 by changing the PianoRoll's connection from m_timeline to &Engine::getSong()->getTimeline(Song::PlayMode::Song), and additionally adding the signal positionChanged to the Timeline class.

The new signal is necessary because getTimeline() returns a Timeline, which does not have a positionChanged signal, while m_timeline is a TimeLineWidget which does.

~~This pr is marked as a draft, since after fixing the bug I found another one: When pressing stop after record-play, the piano roll play head does not go back to the start, but instead stops at its current position.~~ Edit: I have decided to leave fixing that bug to a future pr, as it appears that it existed prior to the regression. Instead, this pr simply fixes the pianoroll playhead movement.