Make drawings part of the model

[martinRenou]

The fact that the drawings are not part of the widget model brings many issues:

• ipycanvas does not work well with jupyter-sphinx and nbconvert
• it will not work well with an ipywidget RTC

We could make the drawings part of the model without altering performances, following the implementation we made for ipympl: https://github.com/matplotlib/ipympl/pull/376

[martinRenou]

We need a way to limit this model size though. Nothing would stop the user to make an enormous amount of drawings.