Fix #18941: Styled barlines before signatures

[XiaoMigros]

Resolves: #18941

This PR introduces style settings for drawing barlines in front of key signatures and time signatures: Either to always use single barlines, always use double barlines, or to use double barlines in front of a system change. The default settings haven't been changed so the behavior is the same.

[its-not-nice]

It might make sense to put these new options in the new 'Clefs, key & time signatures' section now that #20911 has been merged.

I'd envisaged the option being expressed something like "Double barline before key|time signatures" with options "Never"/"Always"/"At end of system only". In particular, the "Single barline" option is a bit misleading as it really means "don't change the preceding barline" (which could be anything).

There is also a question about cautionaries. Currently, double barlines will be drawn where the relevant "At end of system" option is chosen only if a cautionary is shown there; if you turn "Show cautionary" off, a single barline is drawn. We can either:

• keep the same behaviour, and be more specific about the wording of that option ("Only before cautionary [at end of system]" or something), or
• make this option more granular (perhaps "At end of system only" with then an additional checkbox for "Only when cautionary is shown" - or something like that)

(These are questions for @bkunda)

[bkunda]

Very nice feature. Thanks @XiaoMigros!

[bkunda]

@XiaoMigros after discussing this PR further with @oktophonie, we feel this UI arrangement might more accurately reflect what's really going on with double barline behaviour:

Further, I'm fine with these settings remaining under the "Barline" page, so they can stay there for now.

@oktophonie is there anything else we need to add here?

Pls let me know when this is ready for a further review.
Thanks so much!

[XiaoMigros]

@bkunda All set for another review!

[bkunda]

Re-tested on macOS.
All looks good to go from my perspective!
Thanks so much @XiaoMigros