in multivoice, filling rests should not always be added at the end of bar

[matevz]

Steps to reproduce:

1. voice 1: 4 quarters
2. voice 2: 4 quarters
3. place a barline at the end
4. now insert half rest before first quarter in 1st voice
5. rests synchronization occurs filling the bar, but the bar in 2nd voice is filled at the end instead of where the rest was inserted in 1st voice. This is unintuitive, because then you cannot easily shift the 2nd voice for the rest duration.

Expected results:
Voice synchronization should consider place where you inserted rests to also fill the missing rests at that timeStart in other voices. Maybe, find an intuitive way to shift 2nd voice?