Support two-way communication in ipywidgets

[seeM]

Currently, if you create a widget with ipywidgets:

import ipywidgets as widgets
a = widgets.FloatSlider()
display(a)

And adjust the slider, the state is not updated in the kernel:

print(a.value)
# 0

Similarly, if you set the state in the kernel, the widget view doesn't update:

a.value = 100
# Nothing happens in the frontend

This is because we're not handling messages over the widget comm between its frontend and backend.