[Feature Request] Add property Graph.extendFrames to support Plotly.addFrames

[Jerry-Ma]

Hi, I was looking into ways to do graph animation with dcc.Graph, and realized extendFrames would be a good thing to have to avoid sending back the current frames as state.

I never looked in detail of the code but I would imagine we can reuse the same logic used in extendData functionality.

Again, thank you all for such a great package -- the more I use it, the more I love it.