Image node refactor

ve.ce.ImageNode.js
* Moved in generic stuff from MWImageNode
* Added drag end handler (empty, will be used soon)

ve.ce.MWImageNode.js
* Changed to inherit ImageNode
* Moved generic stuff out

ve.dm.ImageNode.js
* Added attribute extraction/preservation for src, width and height

ve.dm.MWImageNode.js
* Changed to inherit ImageNode
* Re-using ImageNode's attribute handling to extract/preserve attributes on both the image and wrapper level

Change-Id: Ied4e1ece24e6804220eac35330790f7084df55de
Merge "Image node refactor"