Image2 in D8

[Olek Nowodziński]

There's a couple of things to be done to match D8 requirements:

1. The internal form of the widget got to be styleable: instead of inline styles, we need to introduce classes.
   1. The non-captioned image has no class.
   2. The class of captioned image must be configurable via config.image2_captionedClass
   3. Tons of tests to be aligned to the new API.

2. We should guarantee that additional attributes (e.g. data-*) are preserved, regardless of the internal state of the widget. Extra attributes must survive any kind of internal processing and re-emerge in editor's output (1:1).
   1. Quite likely it's already working because the image node in DOM is passed along with states of a widget (needs tests).
   2. Alternatively, attributes can be serialized in upcast, saved in widget's data and exploded while downcasting. But let's stick to (a) as an easiest "solution" first.

3. Our static caption placeholder (the "Caption" text) is no solution for Drupal because e.g. it must disappear once editable gains focus. It is possible to implement that with CSS but we need to understand limitations of such approach (i.e. browser compatibility).