robustify button config API:
robustify button config API:

- add fallbacks to 'title', and 'toggle' and 'icon'
- throw error when 'click' handler isn't given

etpinard/plotly.js,iongroup/plotly.js,etpinard/plotly.js,aburato/plotly.js,iongroup/plotly.js,aburato/plotly.js,plotly/plotly.js,asolagmbh/plotly.js,iongroup/plotly.js,plotly/plotly.js,plotly/plotly.js,asolagmbh/plotly.js,plotly/plotly.js,asolagmbh/plotly.js,etpinard/plotly.js