optionsToClasses: Concentrate deprecated code in a single extension
Popup: Add classes option

Fixes gh-7686
Popup: Update enhancement test to check placeholder structure class
Popup: Add classes option to arrow extension
Popup: Add keys "ui-popup-hidden" and "ui-popup-truncate"
Popup: Reflect default style options in default classes option value
Popup: Sync style options to classes option before instantiating

Also, update classes option when style options change