Multi-measure repeats should not break (by default)

The 'keep measures on same system' layout item was originally implemented to prevent system breaks occurring within 2- and 4-measure repeats. (It subsequently was co-opted to squeeze arbitrary ranges of bars onto a system but this is no longer necessary since https://github.com/musescore/MuseScore/pull/25597).

Multi-measure repeats should just not break, by default, without requiring the special use of this layout item to prevent it from happening.

Possibly, in order to allow this to be overridden in specific cases, we could make this a property of the multi-measure 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. There are also some interaction questions, for example about preventing breaks being added within a mm-repeat range, and how we communicate that.)