Introduced data-cke-widget-upcasted attribute to avoid recurring upcasts.
Changelog entry.
Merge branch 't/11533'
config.captionedClass config option to control CSS class of captioned image.

Conflicts:
	plugins/image2/plugin.js
config.captionedClass considered in widget's template.
config.captionedClass considered while upcasting and checking for centering wrappers.
config.captionedClass considered in ACF rules of widget definition.
Updated default classes of captioned images in samples.
Set widget alignment with classes.
Center widget with a class.
Updated doc strings.
Aligned downcast function to latest API.

Conflicts:
	plugins/image2/plugin.js
Removed display style from figure's AC list.
Fixed: Misplaced resizer because of line-height in non-captioned widget.
Simplified class names.
Added comment explaining states of the widget.
i18n of figcaption placeholder made possible.
Extended dev sample to check whether custom attributes survive state shifting.
Commented a mysterious CSS property.
config.captionedClass used in CSS selector to control widget style.