playlistproxy: only emit layoutAboutToBeChanged when changing shuffle mode
Reason for the change
If the playlist was empty and shuffle mode was changed we would previously emit layoutAboutToBeChanged and do nothing, so the matching layoutChanged signal was never emitted. Now we only emit layoutAboutToBeChanged if we are actually about to shuffle/unshuffle the playlist.
Fixes the following tests in mediaplaylistproxymodeltest:
- testSaveAndRestoreSettings
- testShuffleMode
Test plan
ctest --output-on-failure -R mediaplaylistproxy