Implements persistent state of piano keyboard (PianoTools class)

*) PianoTools is unconditionally created at startup (like palette and inspector), so that saveState() and restoreState() can manage it
*) Shown/hidden state is stored into / read from QSettings
fix #20954: Brackets drawn incorrectly in Gonville
update note groups dialog
Merge pull request #327 from mgavioli/PianoKeyboard_persistent_state

Piano Keyboard: persistent state
remove playpanel dependency from sequencer