Issue 4131/1: Reimplement forced partcombine decisions via context properties

Issue 4131/1: Reimplement forced partcombine decisions via context properties

One music type and concept less to worry about.

Since this tracks the partcombine decisions via context properties,
using one forced partcombine override in one voice will not be cancelled
by another forced partcombine in another.

In the event stream, \once\override is indistinguishable from
\override ... \revert so in order to be able to implement
\once\partcombine ... in one voice, we have to retain any permanent
\partcombineForce in the other.