moving, checking MathJax typesetting to WidgetView

widget rendering should not fail hard if MathJax is not available
also setting text in widget.typset
reversing order of arguments, as text may already exist
moving typeset to DOMWidgetView
accept node or selection
moving typeset to utils, usage in cell and outputarea
Merge pull request #6948 from bollwyvl/graceful-mathjax-widgets

Ignore failure of widget MathJax typesetting