The CrossFrame class was a thin wrapper around the Bridge connection
between guest and sidebar and wasn't really encapsulating any
functionality.

Simplify the code by removing this non-abstraction and moving the Bridge
and AnnotationSync construction directly into the Guest class. Also
reorganize the Guest constructor to better group together related logic.

In the anchoring integration test the CrossFrame stub has simply been
removed rather than replaced because it isn't necessary. As long as
`Guest.connectToSidebar` is not called, all the Bridge method calls will
have no effect.