Two settings related fixes

Thanks to #4172 I've found a bug in the code of ISettingsHandler with the OnSettingsSaved() method. It doesn't make sense to define that method as const as it is used to read the changed settings and adjust something or to save additional settings (as is the case with profiles.xml). This fixes saving the master lockcode in profiles.xml after it has been changed in Settings -> System -> Master lock.
Furthermore the "masterlock.lockcode" setting is defined as a string setting (because I thought that's where the master lockcode is stored) but it should actually be an action setting as the master lockcode value is stored in profiles.xml and not in guisettings.xml.

Furthermore I've noticed that when <<loglevel>> is set in advancedsettings.xml we hide the "debug.showloginfo" setting but we didn't hide it's child setting "debug.setextraloglevel" which is then disabled and can't be accessed anyway.