Add symbols to barlines
It sometimes comes up that the best way to solve some particular problem would be to attach a symbol to a barline (eg, think about the commonly-requested multimeasure repeat symbol). But we don't support symbols on barlines, so you end up settling for attaching to the first note or rest in the bar then moving manually.
We already support fermatas on barlines, it's trivial to add support for symbols (and mages too, why not). Only issue is that we don't really handle linking of barlines between score and parts, I guess due to their generated status. I resisted changing this in my PR https://github.com/musescore/MuseScore/pull/4843, but maybe that should be revisited. At least my code there doesn't break if the links show up, and really, we can't depend on links for the types of things my code is doing, so really, that code is good even if we add links.
I have the code to add the symbols, but I think there is no sense making a PR until I deal with the link issue, and that may end up waiting.