LPS-50323 Add documentSelector button and use it in AlloyEditor

Hey @natecavanaugh,

I've fixed some of the formatting issues you mentioned in https://github.com/natecavanaugh/liferay-portal/pull/2835#issuecomment-56994670 (file name, constants on top), but as discussed, keeping the `A.Base.create`.

In order to make `git sfm` happy, I had to write:
```javascript
var CKEDITORTemplate = CKEDITOR.template;

instance._imageTPL = new CKEDITORTemplate(instance.TPL_IMAGE);
```

Otherwise, it will complain about a constructor not starting with uppercase. Does it make sense to have code like this, or should we just ignore the tool warning?

Thanks!

/cc @ipeychev @sergiogonzalez