Add a "Review Mode" editor variant

This PR brings some new frameworking to WebODF, including a `SessionConstraints` class from some of my previous work; the aim is to have an editor variant which lets you annotate a document (perhaps even collaboratively in an OT-rich future), and edit it (both text and direct formatting) in your own annotations and nowehere else. Selection and cursor movement is still possible elsewhere.

`SessionConstraints` allows you to "register" constraints (or permissions, or features -- a better name would be nice) and toggle them on/off in an existing session without requiring an editor reboot.

Every relevant controller now "registers" a constraint called `reviewMode`, set to `false` by default. Having each controller do this ensures as a practice that a controller can 'declare' the constraints it will need. Registering the same constraint multiple times across several controllers has no adverse effect; the constraint state is shared.

The enabling of a `reviewMode` constraint tells a controller to only generate operations for various kinds of edits if the cursor is within an annotation that has the same author as the cursor's owner.
Do we really want to fire `enabledChanged` signals all the time?