NodeList ported to Gee.Collection

[Daniel Espinosa <esodan gmail com>]

At serialization branch, you'll find a port of NodeList to Gee.Collection; replaced GListNodeList with an implementation of NodeList using Gee.ArrayList.

Removed most unused NodeList. I've make some method public, because are used in test cases.

May we need to move most internal classes to be public, but hidden by the developer; while we move most GXml classes to interfaces. These in order to have one or more implementations of the same object (like Document and Element), one to use libxml2 (using classes with Lxml prefix -like LxmlDocument and LxmlElement)

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