fix #41846: incorrect transposition of C key signatures

Two sort of related bugs fixed by this.  One has to do transposition of initial C major key signatures on toggling of Concert Pitch - this was being handled incorrectly, as shown in the initial bug report.  The second has to do with mid-score changes to C being lost completely upon toggling Concert Pitch, as shown in my comment #1.

As fragile as this code has been, I of course welcome review.