Implement proper 'keep on same system' functionality

[oktophonie]

As discussed in #20773 and elsewhere, the 'Keep bars [measures] on same system' item causes some problems. Originally introduced for the purpose of keeping together 2- and 4-bar repeats, it has since become abused to achieve other things.

In the first case, this "Keep bars..." element should not be cloned to parts, since it becomes applied to all parts, when it would only be wanted for the instruments which actually contain the repeats: see #21212

I don't think we should abolish it completely, as there's some value in a 'do not break here if possible' marker (a sort of 'anti-system-break'), but it should be reserved for very specific situations and its use should be avoided in others.

For multi-bar repeats: these should just not break by default. Possibly, in order to allow this to be overridden in specific cases, we could make this a property of the multi-bar repeat items that can be turned off; this property would transfer to parts, but could then also be overridden there as necessary. (We'd need to work out the engraving consequences for allowing these to span a system break, but everything's possible.)

For the purpose of keeping certain bars together on a system, we should have new functionality to do so (by selecting a range of bars and choosing a menu option, clicking a button, whatever). Design and interactions to be investigated, but should achieve the purpose of creating a system containing a specific range of bars (and squeezing to fit if necessary).