on_change callback functions

[markusdr]

It would be useful to have a callback mechanism:

def fct_expanded():
    print('Event detected: expanded')

def fct_checked():
    print('Event detected: checked')

tree_select(nodes, on_change_checked=fct_checked, on_change_expanded=fct_expanded)

This would be analogous to the on_change on st.checkbox (https://docs.streamlit.io/develop/api-reference/widgets/st.checkbox) and other components.