WIP: move event handlers to client code

[hansthen]

First step, move (some of) the event handlers to python code.

Eventually, I'd like the users to add the event handlers themselves (and have streamlit-folium provide a set of standard event handlers).

I am thinking of removing the standard set of returned_objects and replace it with a typed event structure. (So that streamlit-folium returns events, similar to https://github.com/null-jones/streamlit-plotly-events)