SkinAction should use the first XWikiSkins object found in the document, not just the one with number 0

Currently, it gets the object with:
doc.getObject("XWiki.XWikiSkins", 0);
which forces the object with number 0 to be considered. However, if a user deletes that object an creates another one, it will have number 1, and thus it will never be possible to use it.

Instead, it should use the getObject method that does not have a number argument.