Add review editor to programs/editor
Rename SessionContextCache to SessionContext.

Stops caching cursor range.
Rename imageInsertingEnabled to imageEditingEnabled
Hide annotation removal buttons of other authors
when the edit.annotations.onlyDeleteOwn constraint
is enabled.
Add a const `gui.CommonConstraints`, which lists
the common constraints that come with and are
respected by WebODF.

Use these constraints in all previous occurrences.
Change widgets that access controllers to cleanly
unsubscribe and subscribe to signals.
Add a readme for constraints