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

Improve deletion over annotations and anchors
Refactor HyperlinkClickHandler to cleanup event binding

Move control of event bindings from SessionController to
HyperlinkClickHandler so bindings will be updated whenever modifier is
changed. This allows some simplification as HyperlinkClickHandler can
simply use the KeyboardHandler.Modifier.
Add tooltip support for hyperlink