Made widgetDefinition specific for the editor.
Made upcastWidgetElement method specific for the editor.
Made isCenterWrapper method specific for an editor (now centerWrapperChecker).
Improved detection of centering wrapper in centerWrapperChecker.
Fixed: Possible upcast of non-captioned widget with DIV centering wrapper in ENTER_P.
Aligned widget downcast to avoid failures in modes different than ENTER_P.
Performance: Don't create objects when it's not necessary.
Use enterMode instead of activeEnterMode since dataProcessor is not yet contextual.
Changelog entry.
Merge branch 't/11283b'