The skin class should have distinct 'parent skin' and 'directory' fields

Currently, XWikiSkins has a "base skin" field, which can point to either another XWikiSkins document, or to a directory on the filesystem, which makes it hard to have a parent skin which is overridden by files in a directory.

The normal behavior would be to have a "directory" property which indicates where are the skin files stored, and a "parent skin" property which indicates where should the engine search for skin files not found in the directory.