Renamed a few methods to be more consistent

(As previously discussed with @tilgovi [here](https://github.com/hypothesis/h/pull/1735#commitcomment-8799722).)

Sidebar -> Document RPC methods:
 * Renamed `scrollTo()` to `scrollToAnnotation()` (to be consistent with `focusAnnotations()`

Guest methods:
 * Renamed `showViewer()` to `showAnnotations()` (to be consistend with `focusAnnotations()`)
 * Renamed `updateViewer()` to `updateAnnotations()` (same reason)
 * Renamed `toggleViewerSelection()` to `toggleAnnotationSelection()` (same reason)