Custom barlines behavior

I use custom barlines quite a lot in my music. I tried the latest trunk feature based on Miwarre's excellent work, and found some behavior quite confusing, sometimes buggy (see #26416 and its comments).

Some examples of confusing, strange or buggy behavior:
- when you drop a tick, it appears but is not saved
- when you change the span of a tick it makes all other barlines a tick
- when you drop a tick and edit the double barline at the end, all barlines will be the same height
- when you drop a tick, then create a new measure, the new measure is created with a tick
- when you add a double barline, then turn it into a tick, it is saved
- when you pull a full/double/etc. barline on a tick, only the type is changed and not the span, so a tick-size double bar will appear (see 26416)
Actually it would be very hard to fully describe all current behavior, and explain, why it is bad, so below I try my best to suggest an understandable and implementable consistent behavior.

So here I would like to start a discussion about how it should work, then I'd happily offer my help to fix it in the code.

As the basis of the discussion I suggest the following aspects:
- frequently used stuff should be super-easy
- rarely used stuff can be complicated (ie. available only from the inspector)
- try to follow conventions from Sibelius (assuming they had some extensive user testing)

So based on this I have the following recommendation:

1. A normal barline can span through or between staves. Setting this span would cause the all normal barlines in the given staves span according to this setting. I call this kind of span "big span".
2. Using the barline handles should only affect "big span" - similar to Sibelius and previous versions of MuseScore. If you want to change the "small span", ie. the spanFrom and spanTo lines, you should use the inspector.
3. If someone needs a tick or short barline (ie. change the small span), that should affect only the given staff and measure, making other barlines on other staves reset to default "big span".

So as (probably not exhaustive) examples:
- when dropping a normal barline or repeat or dashed or dotted (etc) on a measure that should not change the big span of the barline (so would stay 1-2-... staff high or mensurstrich).
- when dropping a tick or short barline from the palette on a measure, that should only change the barline for that measure in that staff. If the staff has mensurstrich, the big span changes to no span for the given staff (and the neighbor staff)
- when dropping a barline onto a note the barline should not be affected with the big span at all
- changing the "big span" is global, ie. if you edit and drag a normal barline (only those have drag handles!), all normal barlines will span according to the new setting.
- changing the "small span" is local - to locally have mensurstrich e.g. you can use the small span (or a very big number could automatically be drawn to the next staff)

That is, we should minimize the global effects, and make very clear what edit has and what edit doesn't have a global effect.

So I would like to have a discussion here: what do you think?