Fix #20931 - State of Inspector widget was not remembered across MuseScore sessions

Fixed by:
*) Storing in the settings the state of the Inspector widget (dock state, position, size) at close down
*) Unconditionally creating the Inspector widget at srt up (as for the F9 palette widget) before settings is read
*) Restoring the Inspector state from settings at start up
Merge pull request #322 from mgavioli/Fix_20931_remember_inspector_status

Fix #20931 - Remember state of Inspector widget