Change the relationship between the `Guest` and `Sidebar` classes from
one of inheritance to one where `Sidebar` receives a reference to the
`Guest` instance and calls methods on it or subscribes to events from
it. The `Guest` also no longer "owns" references to the bucket bar /
toolbar objects that logically belong to the sidebar. Instead it emits
events which the sidebar responds to.

This change makes the interface of the `Guest` used by the `Sidebar`
more explicit and ensures a better boundary between the two. This also
makes it easier for the `Sidebar` tests to be concerned only with the
interface of the `Guest` and not its implementation details. In future
this change will also make it possible to have a frame which does not
contain a sidebar but is not annotateable. We had a need for this
historically when integrating with epub viewers, although it was never
implemented.

Updating the tests for `PdfSidebar` was complicated by the fact that the
`PdfSidebar` tests are not pure unit tests. They instantiate the
`Sidebar` base class and so depend on many implementation details of it.
To make this change and others easier, the `PdfSidebar` tests have been
changed to mock the `Sidebar` base class. The steps involved in this are
non-obvious so I extracted the logic into a utility function.

 - Change `Sidebar` to no longer inherit `Guest` but accept it as a
   constructor argument
 - Remove direct references to the `BucketBar` and `ToolbarController`
   instances from the `Guest` class and instead emit events from the
   `Guest` which the `Sidebar` responds to.
 - Add a `mockBaseClass` testing helper in `src/test-util/mock-base.js`
   and change the `PdfSidebar` tests to use it, so that they are less
   coupled to `Sidebar` implementation details.