Further refactoring to Selector mixin

* Fix a bug related to multiple columns
* Move more functions into the prototype so they can be overridden
* Incorporate fix for #822
Additional Selector tweaks and test cleanup

* Use late binding for maximum extensibility
* Move selector check outside of _configureSelectorColumn
* Add flag to avoid needlessly calling _updateHeaderCheckboxes
* _changeInput -> _changeSelectorInput
* Minor reformatting
* Update remaining instances of selector: true to "checkbox" in tests
* Remove grids in test page showing spurious use cases with checkbox + radio
Tweaks to Selector docs