VNC backend update

- Add IPython DOMWidget to _ipynb_vnc. Now, backend has its own widget to display.
- Edit setup.py, .js files will appear in build.
- No need to write Js code into notebook for event handling.
You want to add here dummy variables for what is used below. It must be possible to import this module even if it cannot be used:

    DOMWidget = object  # Dummy objects
    Unicode = Int = Float = Bool = lambda *args, **kwargs:None