Initial image feature implementation

[szymonkups]

Implementation should follow Editor Recommendations: Image.
Image will be the first widget-based feature, and should be a reference implementation for other widget features to follow.
Topics connected with Image feature and widgets:

• fake selection - discussed here https://github.com/ckeditor/ckeditor5-engine/issues/627,
• D&D - image and other widgets should be draggable,
• nested editables should be supported (to implement for example image caption),
• things like captioning and alignment should be easily reusable in other widgets implementations.

UX and UI of image feature will be discussed later on.

[szymonkups]

I've created PR ckeditor/ckeditor5-image#7. It includes very basic image support (see PR description). Further implementation will be done in separate issues that will address things described in previous comment:

• D&D - image and other widgets should be draggable,
• nested editables should be supported (to implement for example image caption),
• things like captioning and alignment should be easily reusable in other widgets implementations.