[WIP / POC] Ability to prevent initial callback from firing

[chriddyp]

Via a new property in the app.callback, prevent_initial_call=True

@app.callback(Output('another-output', 'children'), [Input('another', 'value')], prevent_initial_call=True)

This will prevent it for both initial renders in app.layout as well as new component renders from a callback.

WIP / POC for now. Would need lots of tests.

[christianwengert]

+1 I believe this would be very useful. With the wildcards it can happen very quickly, that I have like one hundred initial callbacks which make the app very unresponsive for a couple of seconds. By preventing all wildcards to fire this would be much faster and better

[alexcjohnson]

superseded by #1225