adding mid-score key signature should affect all staves by default

For tonal music, it essentially *never* happens that we need to change keys in one instrument but not others. Key signatures, like time signatures, should only need to be set once, not separately on each staff. Wanting different key sigs for different staves is a very rare exception (except for transposing instruments, of course, but that's not what I'm talking about). As it is, the fact that key signs need to be placed with drag&drop, and then need to be repeated for each staff, makes modulations in orchestra scores far more painful than they should be.