Add a public API to retrieve a XWikiPreference with a fallback parameter on xwiki.cfg and a final default value

The signature would be :

public String getXWikiPreference(String prefname, String fallback_param, String default_value);

The underlaying method already exists in the non API XWiki.java (was added for http://jira.xwiki.org/jira/browse/XWIKI-2569).

What this method exactly do is : "Obtain a preference value for the wiki, looking up first in the XWiki.XWikiPreferences document, then fallbacking on a config parameter when the first lookup gives an empty string, then returning the default value if the config parameter returned itself an empty string."