[Bug] Bind handleAdd so that implementors can get a reference

I've marked as a bug just because I think this reflects a minor inconsistency; but it could also be understood as a feature request.

The scenario is:

• I'm providing my own methods for uploadFile and handleAdd, which is something that's possible today.
• I can see that uploadFile has been bound to its parent object, allowing me to get a handle of the editor object by doing: this.config.em.get('Editor') inside my method, which is cool.
• But, I cannot do the same for handleAdd. I'd like to be able to do the same - and it'd be more consistent, I believe.

As an alternate solution, the editor could be passed as the last param of both those methods - I'm fine with that as well, let me know if that's preferred instead.