Add a shutdown hook in Zotero.Prefs to unregister observer
Add a shutdown hook in Zotero.Prefs to unregister observer

* Without this, preference change observers stack, triggering multiple
  times if Zotero is re-initialized multiple times within the same
  browser instance (like in tests)

gracile-fr/zotero,gracile-fr/zotero,gracile-fr/zotero