Bug 1797156 - Refactor OverflowableToolbar and add documentation. r=sclements,cmkm

This patch does not aim to change the behaviour of OverflowableToolbar
at all. Instead, it:

1. Migrates off of old-school prototype class to a modern ES6 class
2. Uses actual private members and methods rather than underscore
   pseudo-private members and methods.
3. Tries to group the methods of OverflowableToolbar into groups like
   public methods, private methods, private event handlers, CUI widget
   listener methods and finally XPCOM interface implementations.
4. Tries to add JSDoc-style documentation where possible.

Differential Revision: https://phabricator.services.mozilla.com/D160106