Ability to configure defaults for Canvas

[FeldrinH]

As shown in the example notebook, 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.