HTML Widgets; widget.GetAgeInYears method added

A new widget.GetAgeInYears JavaScript function has been added.

This is passed a date of birth and, optionally a date as at, e.g.:

var years = widget.GetAgeInYears(new Date(1969,8,23));

var years = widget.GetAgeInYears(new Date(1969,8,23), new Date(2019,1,23));