Make Xinha more extendible

We created our own XinhaNodePlugin and came across these extendibility issues:

• AbstractXinhaPlugin has inner classes with package visibility (Configuration, EditorManagerBehavior) so I had to override this class as well
• We extended XinhaImage and ImageBrowserDialog; ImageBrowserDialog's constructor has argument IModel<XinhaImage> model which may be wildcarded to IModel<? extends XinhaImage> model