Attempt to simplify the signature of the PDFSidebar constructor, by moving the eventBus parameter from the options object and removing the PDFOutlineViewer dependency

This is similar to the format used by a number of other viewer components, and should simplify the `PDFSidebar` initialization slightly.
Furthermore, by using the `eventBus` it's no longer necessary for `PDFSidebar` to have a direct dependency on `PDFOutlineViewer`.

There's still room for improvement here, but this patch is at least a start (since it's not clear to me how best to handle the viewers).

web/pdf_outline_viewer.js

@@ -39,6 +39,8 @@ class PDFOutlineViewer {
     this.eventBus = eventBus;
 
     this.reset();
+
+    eventBus.on('toggleoutlinetree', this.toggleOutlineTree.bind(this));
   }
 
   reset() {