Allow customization of the locations where persistent settings are stored.

Allow customization of the locations where persistent settings are stored.

Make it possible to set a non-default wxPersistenceManager to use and allow
overriding of GetConfig() and GetKey() methods by making them virtual and
documenting them.

This can be notably used to allow porting of the existing code to use
wxPersistenceManager while keeping compatibility with the old settings.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69583 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775