Allow node focusing

*.php
* Added links to new file

ve.ce.ImageNode.js
* Added focusable node mixin

ve.ce.FocusableNode.js
* New class!
* Adds isFocused and setFocused methods
* When a node is focused or blurred, 'focus' and 'blur' events are emitted
* While a node is focused, it will have the 've-ce-node-focused' class added to it's this.$

ve.ce.Surface.js
* Add detection of node focusing and setting focus and blur on nodes on change

Change-Id: I3f1ad6309571f2bfe568550e2e8f1bd5a0302085
Merge "Allow node focusing"