Refactor AdvancedSettings usage by making it an IDisposable instance mod...

...ifying global static (i.e. 'Shared') data. The disposing (via a Using block) ensures that Save is only made once per Using block instead of per method call to SetXXXSetting or ClearXXXSetting. Also made some of the methods/nested classes that weren't being used Private.

I left the read functions as 'Shared' functions since they don't need any kind of special handling at the moment.