Improve and simplify title compatibility code for relayout [882]
Improve and simplify title compatibility code for relayout [882]

- Downsize the cleaning of relayout attributes to one method.
- Allow titles to be numbers, not just strings.
- Call cleaning method just in one place.
- Make comments more clear.

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