updated metadata for schemas
added preference dialog and schemas
[prefs] enabled saving of data update time
[prefs] enabled saving of Unit
[prefs] enabled saving display-degree-sign and display-decimal-value boolean
[prefs] enabled saving of show average/maximum in panel
[prefs] added display-hdd-temp boolean in preferences
enabled checking of 'display-hdd-temp' preference
implemented 'update-time' preference
enabled all preferences
added "Preferences..." item on popup menu