[MU3] Harp and braces

[njvdberg]

Resolves: https://trello.com/c/iVfSQodt/15-harp-has-all-the-brackets

Solves a bug preventing generating square braces for an instrument having multiple staves.
Next moved the harp to a dedicated family for the Orchestra score order.

• 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