Enable Virtual Selections in Viewer

This PR uses a simple way to enable virtual selections in the viewer by initializing all the required editing modules, just falling short of calling `sessionController.startEditing()`.

This lets the shadow cursor and normal local cursor be movable by mouse or keyboard.
An additional `ODFViewerPlugin.css` file is added (just like the corresponding css file for the PDF backend in ViewerJS) that hides the caret and avatar handle for the local cursor.

Also fixes another issue: the `class` attribute of the avatar handle div was being used to mark the focus state. This state is now expressed by an `focus` attribute that can take the values `"active"` or `"disabled"`. The `class` is reserved for the `"handle"`.

I do not consider this solution optimal; I'd rather enable the VS/cursor movements through a constraints system. But till that is really ready, this ought to do.