API Changes on GXml.Node

* Added new GXml.NodeList as Gee.ArrayList<GXml.Node> replacing
  old GXml.NodeList based on xNode, renaming it to GXml.xNode
* Added new GXml.ElementList as Gee.ArrayList<GXml.Element>
* New lists types helps on bindings
* HtmlDocument and GXml.Node search for properties return a list
  of GXml.Element instead of GXml.Node ones
* Fixed tests for HtmlDocument API
* Removed warnings