Add CodeMirror XML editor widgets for levelbuilder block edit form fields

Screenshot:
![image](https://cloud.githubusercontent.com/assets/56541/3707963/4a1149e8-143c-11e4-9d03-f7ebcbe2e77b.png)

Also:
* Separate level-editor javascript/css dependencies from main 'application' bundle into 'blockly_editor'
  * (reduces file size of main application js/css bundle download)