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.
The symlink fails on Windows, on both Python 2 and Python 3. Would it be okay to use `symlink=True`? (ping @Eric89GXL @almarklein)