fix dataTable customization points

[gordonwoodhull]

As noted here, the customization points in the data table are different from usual. The only other place where we expect users to overwrite a method to do normal customization is doClick.

Instead there should be getter/setters. It's great that these all are customizable:

• _doColumnValueFormat
• _doColumnHeaderFormat
• _doColumnHeaderCapitalize
• _doColumnHeaderFnToString

... but these should be getter/setters

• formatValue
• formatHeader
• capitalizeHeader
• stringifyHeaderFn

Two options:

1. Backward-compatible. Keep the public, customizable members but add getters/setters.
2. Purist. Make them private members, either ignore the old names or make assignment to the old names fail using defineProperty. (Could even deprecate the old setters with a warning, hmmmm.)