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.
Merge pull request #451 from peitschie/delete-over-annotation

Improve deletion over annotations and anchors
Split Editor into woeditor and wocoeditor

* create needed editor structure divs ourselves
* use classes for styling of editor structures
* use namespaced, numbered element ids to allow more than one editor
move fullscreen pinch-zoom into own class FullWindowZoomHelper
make all editor toolbar actions configurable
Add about dialog to editor component