Let the Editor create its used div subtree itself

* use classes for styling of editor structures
* use namespaced, numbered element ids to allow more than one editor
Merge pull request #461 from kossebau/editorCreatesOwnDivs

Let the Editor create its divs itself
Improve backspace and delete behaviour to function over inline roots

Backspace and delete characters were "sticking" when encountering other
roots in the document (e.g., annotations). Improved logic uses step
iterators to improve the robustness of this calculation.

Partially fixes issue #56.