Improve the XWiki implementation of the XML-RPC Confluence API

The XWiki implementation of the XML-RPC Confluence API is deficient:

• Not sufficiently tested (XWIKI-1528)
• Incomplete (XWIKI-1559)
• Not interoperable with Confluence (XWIKI-1616)
• Poorly designed (lots of duplicate code) (XWIKI-1617)
• Without proper exception handling (XWIKI-1618)
• Insufficiently documented (XWIKI-1626)

We plan to improve it on all these fronts.

We are also working on extending the XML-RPC Confluence API where needed (XWIKI-1664)