Investigate and add support for the @callback decorator of Dash 2.0

[delsim]

Dash 2.0 introduced a new decorator @callback

This appears to be ignored by the current implementation of django-plotly-dash and we should investigate and then (if possible) support this form of decoration.

This issue has been reported in #452

[sondalex]

Hi, I am wondering whether there are any advancement on the integration of the @callback decorator from Dash 2.0 It would be greatly beneficial to be able to use AIO components without having to redefine default callbacks with the DjangoDash().callback decorator.