Let the Editor create its divs itself

Looking around I found that usually the id of an existing element is passed in, to have the whatever-to-be-created thing then extend that node with subelements as needed. This seems sane, as the caller of the API should not need to know what substructure is used, that is an implementation detail which can be safely hidden away (also allows to change things as needed, without breaking programs.
For current integration needs with ownCloud Documents the constructor of the Editor class accepts an optional parameter ```toolbarContainerElementId```, to refer to an existing div element in a custom structure to be used to place the toolbar in, instead of creating one internally.

This PR also switches the styling of the editor substructures from ids to classnames, to remove another blocker for multiple editor instances in the same address space.

Next step will be splitting the Editor class into a version for single user editing and one version for collaborative session-based ones, because supporting both in one class only complicates things. See the branch https://github.com/kossebau/WebODF/tree/woeditor for a draft of the long-term goal.