Fix #16506 Former key signature isn't restored after Undo Transpose

[heuchi]

When transposing key signatures Score::transposeKeys() updates the keymap in transpose.cpp:417
(*km)[tick] = nKey;
This is not stored in the undo stack.

While undo resets the actual KeySigs, layout will later change them back to match the (old) keymap.
I added a call to setKey() in ChangeKeySig::flip() to make sure the keymap matches all key signature changes.

[heuchi]

added test mtest/libmscore/transpose/tst_transpose.cpp:
transpose test score major second up, compare to reference, undo transpose, compare to original file.

[Jojo-Schmitz]

as per #16506 this has been fixed, so this PR can get closed, can't it?

[heuchi]

I don't see this is fixed. See new issue #23525

[heuchi]

I rebased this from current master. It's addressing issues 23525, 21234 and 22913.

[lasconic]

Thank you for the rebase! Do you mind if I apply the patch instead of merging? The history mentions 2 "users" heuchi and Joern Eichler and I would like to keep the history as clean as possible. You could also make a new PR if you prefer.

[heuchi]

I just saw this and am trying to solve it using a git filter-branch. If that's not a good idea, please tell me. I seem to not having set up correctly my git system here.

[heuchi]

OK, this went wrong... I'll create a new PR.

[lasconic]

I have no experience with git filter-branch but it looks like a weapon of mass destruction :)

[heuchi]

New PR: #540