The various top-level Hypothesis UI elements in the annotator have a
common structure where:

 - There is a `<hypothesis-{name}>` container element

 - The container has a shadow root with the annotator stylesheet loaded
   into it

 - Preact is used to render the contents of the element, into the shadow
   root.

 - There is a controller which wraps the container element and has
   methods to read/write the displayed data. When the data is changed,
   the Preact tree is re-rendered.

Add a `PreactContainer` utility which implements common parts of this pattern,
and refactor two of the top-level UI controls (bucket bar and adder) to use it.