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.
config.captionedClass updated default classes of captioned images in samples.
Updated doc strings.
Fixed: Misplaced resizer because of line-height in non-captioned widget.
i18n of figcaption placeholder made possible.
Extended dev sample to check whether custom attributes survive state shifting.
Improved Image2 style definition in contents.css. Removed obsolete argument.
config.image2_alignClasses defined in the plugin
config.image2_alignClasses considered in ACF rules.
config.image2_alignClasses used in dev sample.
config.image2_alignClasses considered in widget features.
config.image2_alignClasses considered while detecting centering wrapper.
config.image2_alignClasses used to set internal widget alignment.
config.image2_alignClasses detected in widget's init.
config.image2_alignClasses used while creating centering wrappers.
Introduced CKEDITOR.htmlParser.element#addClass.
config.image2_alignClasses considered while downcasting.