This commit re-implements the vertical toolbar on the left edge of the sidebar
to make future changes to the toolbar UI easier (eg. some upcoming a11y changes)
and to decouple it from the rest of the annotator application (the toolbar and
`Sidebar`/`Guest` classes currently access each other's internals in a rather
haphazard way).

The new implementation has a similar code structure to the `Adder` toolbar that is
shown when selecting text. It consists of:

 1. A Preact component in `src/annotator/components/toolbar`
    which renders the toolbar UI according to the current state

 2. A controller class in `src/annotator/toolbar.js` which renders the
    Preact component into a container element and has provides properties
    that the `Sidebar` and `Guest` classes can use to update its state.

    The controller takes only the container element and callbacks
    associated with each button as inputs, so it no longer calls methods
    on the Guest/Sidebar.

    The sidebar and guest now update the toolbar UI by setting properties
    on the toolbar controller, rather than using a mixture of manual
    manipulation of the toolbar DOM and publishing events which get
    eventually translated to method calls on the toolbar.

    If we convert the remainder of the sidebar UI from jQuery/manual DOM
    updates to Preact in future, then we can remove the toolbar
    controller entirely and just render the toolbar component from
    within the sidebar component.