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.
Solved it here: https://github.com/mfkaptan/vispy/commit/fc7ae60c7479ffce5498e8556d9e99ef3cc6e5e8