Widget: Process "classes" option during _setOption()
Autocomplete: Implement _elementsFromFromClassKey()
Accordion: Implementing _elementsFromClassKey()
Button: Implementing _elementsFromClassKey()

Also for buttonset.
Dialog: Add support for setOption with classes
Menu: Add support for setOption with classes
Progressbar: Add support for setOption with classes
Selectmenu: Add support for setOption with classes
Slider: Add support for setOption with classes
Spinner: Add support for setOption with classes
Tabs: Add support for setOption with classes
Tooltip: Add support for setOption with classes
Accordion: Update classes option to use empty string instead of null
Autocomplete: Update classes option to use empty string instead of null
Button: Update classes option to use empty string instead of null
Dialog: Update classes option to use empty string instead of null
Menu: Update classes option to use empty string instead of null
Progressbar: Update classes option to use empty string instead of null
Slider: Update classes option to use empty string instead of null
Spinner: Update classes option to use empty string instead of null