A side effect of using shadow DOM for the sidebar is that the BucketBar
'plugin' could not be injected easily using a current query mechanism.
After consulting with @robertknight, we decided to avoid using the
normal plugin injection mechanism and instead instantiate the BucketBar
from the sidebar.

This PR also includes:

- a no documented configuration option to disable the shadow DOM
  encapsulation. This can be removed in the future if not needed.
- more strict types
- simplification of the logic in the sidebar

I have tested these changes in the following browsers:

Brower\OS    | MacOS              | Windows
------------ | ------------------ | -----------------
Chrome 57    | ✔ | ✔
Chrome beta  | ✔ | ✔
Edge 17      |                    | ✔
Edge beta    | ✔ | ✔
Firefox 53   | ✔ | ✔
Firefox beta | ✔ | ✔
Safari 10    | ✔ |
Safari 14    | ✔ |