Widget: add the _classes method.

Fixes #7053
Tabs: use the _classes method.

Fixes #7053
Dialog: use the _classes method.

Deprecate dialogClass.

Fixes #7053
Accordion: use the _classes method.

Fixes #7053
Autocomplete: use the _classes method.

Fixes #7053
Slider: use the _classes method.

Fixes #7053
Spinner: use the _classes method.

Fixes #7053
Progressbar: use the _classes method.

Fixes #7053
Menu: use the _classes method.

Fixes #7053
Tooltip: use the _classes method.

Fixes #7053
Selectmenu: use the _classes method.

Fixes #7053
Widget: Process "classes" option during _setOption()
Autocomplete: Implement _elementsFromFromClassKey()
Accordion: Implementing _elementsFromClassKey()
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