PR: Change when the sidebar is opened - v2

New iteration of my solution for #649.

This does the following things:
 * Introduce the publicly accessible `showFrame()` and `hideFrame()` methods to the host frame, for opening/closing the sidebar
 * Introduce a new method, named `updateViewer()`, which is almost like `showViewer()`, except that it does not actually open the sidebar, only updates the content of it
 * Started to use this new method (that does not open the sidebar) in the following cases:
   * A new highlight is created
   * Heatmap is updated

Unlike the previous PR (https://github.com/hypothesis/h/pull/650), this does not change the semantics of `showViewer()`.