config.image2_captionedClass config option to control CSS class of captioned image.

Conflicts:
	plugins/image2/plugin.js
config.image2_captionedClass considered in widget's template.
config.image2_captionedClass considered while upcasting and checking for centering wrappers.
config.image2_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.image2_captionedClass used in CSS selector to control widget style.
Moved Image2 style definition to contents.css.