[MU3] Solve brace scaling issues

[njvdberg]

Resolves: https://trello.com/c/knmrYs5s/35-braces-are-too-thick
Resolves: https://trello.com/c/VFE5Ivxj/69-when-resizing-grand-staff-brace-scales-on-the-x-and-y-it-should-only-scale-on-the-y

Use the correct width scaling for braces to prevent a width scaling which includes extra added space between staves enclosed by the brace.

To make sure the palette contains the right symbol, Bracket::setStaffSpan() is used to define the brace symbol and width scaling used in the palette instead setting the symbol using Bracket::setBraceSymbol() which does set the correct symbol but does not set the correct scaling.
Also, the calculation of the scaling includes the correct akkoladeDistance>/code>/maxAkkoladeDistance.

• I signed CLA
• I made sure the code in the PR follows the coding rules
• I made sure the code compiles on my machine
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• I made sure the commit message title starts with "fix #424242:" if there is a related issue
• I created the test (mtest, vtest, script test) to verify the changes I made