This class is a simple wrapper around Foswiki::Prefs::Stack. Since Webs has an
hierarchical structure it's needed only one stack to deal with preferences from
Web and Web/Subweb and Web/Subweb/Subsubweb. This class has a reference to a
stack and the level where the web is.
This class is used by Foswiki::Prefs to pass web preferences to Foswiki::Meta
and should not be used for anything else.