Convert this module, but not its tests, to JS. This is a pretty
straightforward conversion with a few minor improvements:

 - The initialization of `options` was rewritten in a way that is
   consistent with `PdfSidebar` and avoids a non-idiomatic way of
   passing initialization parameters from derived to base class

 - Some comments that were hard to parse were rewritten or removed if
   they stated something obvious

 - Private methods and fields related to gesture state were prefixed
   with an underscore. Some of the sidebar tests did and still do access
   these, which is not ideal.