See #2245

It's quite a common setting to extend the bar lines between staves in a system.

One approach is actually to extend the lines across all staves in the system:

Another approach is to connect not only between staves belonging to a single instrument/track (Musescore allows to manually drag the lines to connect it specifically):

There are two options to make this feature:

1. Add a stylesheet options configuring the mode and then apply this generally
2. Add a flag to the data model (staff) whether to extend the bar line to the next staff. (as possible extension to 1.)