Wikia media caption node

Without creating our own WikiaMediaCaptionNode, the model tree contained:

wikiaBlockImage
mwImageCaption
/mwImageCaption
/wikiaBlockImage

The childNodeTypes and parentNodeTypes were mismatched and created problems with drag&drop (and probably copy&paste). The only changes to core VE remove hardcoded type references and instead refer to the class's static properties (which can be changed via inheritance).