Ability to configure defaults for `Canvas`

As shown in the [example notebook,](https://github.com/matplotlib/ipympl/blob/main/docs/examples/full-example.ipynb) it is possible to configure the appearance of the `Canvas` widget by setting some attributes on it.
A lot of these attributes are settings that I would like to configure in the same way for all figures. It would be very convenint if there was some way to set the default values of these attributes before creating plots (similar to `matplotlib.rcParams`).

I tried changing the traitlet definitions directly (i.e. `Canvas.header_visible.default_value = False`), but apparently the traitlets cache the original default values somewhere and ignore the change to `default_value`.

Start with docs/examples/full-example.ipynb and the Canvas traitlet definitions mentioned in the issue. Identify how attributes such as header_visible are currently configured, then determine a way to set defaults before plots are created; done means newly created figures inherit those defaults without per-Canvas assignments.

Written by the indexing model from the issue text.