Implemented no rhythms mode (bare note head) - it is quite tricky:...

Implemented no rhythms mode (bare note head) - it is quite tricky: no-rhythm-note has duration of 1 (unreal) and meter of no rhythm also has single group with duration of 1, so every measure has single note. Gap factor is 0.75 for no-rhythm-note - so all staff notes positioning/measure shifting works without any other change.
Added rhythmsEnabled option of score params, exposed it to QML as well