annotator: Don't magically load remote scripts

This is not a great idea. It needs to be replaced with a polyfilling
build of Annotator that can be loaded by the bookmarklet if it detects
missing functionality.
annotator: Minor reorganisation
annotator: Expose the UI module at Annotator.UI
notification: Clean up notification component

This makes the notifier a new kind of component like storage, that can
be set at startup. The interface of a notification component is very
similar to that of HTML5 Notification.

The default Annotator is now instantiated with the old ("banner")
notification component enabled.
plugin/unsupported: Update Unsupported plugin and add tests
plugin/kitchensink: Remove kitchensink plugin
core: Minor tweak to language
core: Rename plugin destroy handler -> onDestroy

And, for simplicity, use runHook to run the onDestroy handlers.