Allow node relocation

*.php
* Added links to new file

ve.ce.ImageNode.js
* Added relocatable node mixin
* Added $image reference to the actual img element, so if it's wrapped
  in a sub class the functionality in the parent class  doesn't break.
* Moved drag start event handling to relocatable node
* Removed drag end binding, not needed.

ve.ce.MWImageNode.js
* Moved addClass to initialization section of constructor.
* Copied 'view' data prop from image element to keep stuff working after
  the wrapping.

ve.ce.Node.css
* Switched to default (arrow) cursor for images.

ve.ce.RelocatableNode.js
* New mixing for nodes that should be relocatable
* Added implementation for drag start, which tells the surface to allow
  dragging this node.

ve.ce.Surface.js
* Added relocation support, which is used by relocatable nodes
* Split onDocumentDragDrop into onDocumentDragOver and onDocumentDrop
  which now have implementations that support relocation of nodes

ve.ui.Context.js
* Added relocation tracking to prevent context being shown while
  relocating

Change-Id: I8703adfb707af2c3224431afc3418356ac2c686c