fix #24084: mixer pan not exported to MusicXML
fix #38976
Merge pull request #1531 from lvinken/musicxml-fixes

fix #38976
Merge pull request #1529 from Jojo-Schmitz/pan

fix #24084: mixer pan not exported to MusicXML
deselect after use if nothing was selected before

in AddRemoveBreaks and ResequenceRehersalMarks.
Also prevent ResequenceRehersalMarks to destroy a list selection,
AddRemoveBreaks  prevents that already,