[MU4 Issue] Bracket width is calculated incorrectly when determining left margins

[oktophonie]

Describe the bug
Width of brackets is not calculated correctly when determining left margins, which should be consistent through the score (otherwise implemented in #9277).

Saith @cbjeukendrup: "in this case the brackets start pushing the instrument names off the page, instead of pushing the systems to the right [...] it becomes apparent that the code for calculating the "total" brackets width (i.e. if all staves were visible) does not work at all: it just returns the brackets width for the current system, so the result is that each system has its own left margin so the systems end up not being aligned consistently. "