[MU4] SMuFL Symbol Anchors, ledger lines, flag placing, Shadow Note refactoring

[cbjeukendrup]

• Implement new way to get and set SMuFL Anchors for symbols
  There is a new enum for the currently supported anchors, and the anchors are now stored in a std::map.

• Fix ledger line length for Chord and Ambitus
  That is:
  a) draw with FlatCaps instead of RoundCaps
  b) append the whole ledgerLineLength at both sides of the note, instead of half on each side.

• Correctly handle note flag placing, respecting bbox
  A MU4 version for PR [MU3] Score symbols bounding boxes #6894

• Shadow note refactoring
  In a way a MU4 version of PR Fix shadow note ledger lines and stem length #7064
  Give the Shadow Note more information and make better use of its inheritance from Element, so that is can be drawn much more 'realistically'.
  Also implements the corrections made to flag placement / stem height from the previous commit.

• Fix stem length of notes with mirrored notehead
  A MU4 version for PR Fix stem length of notes with mirrored notehead #7050
  At least, the issue is fixed only after a re-layout is triggered. That could suggest two things:

  • It is broken in another place too (but very unclear where)
  • n->mirror() doesn't return the right value at first.

• 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

[igorkorsukov]

Please could you do rebase

[cbjeukendrup]

Please could you do rebase

Done!