Allow outline to be collapsed / shown via icon.

- This commit adds a '>' before every outline item that has subitems.
- Click on the '>' to collapse all subitems under that item (which turns
  the '>' in a 'v').
- Shift + click expands/collapses all descendant items under that tree.
- Double-clicking on the "Show Document Outline" button in the toolbar
  expands/collapses all outline items.

web/viewer.js

@@ -1401,6 +1401,11 @@ function webViewerInitialized() {
       PDFViewerApplication.switchSidebarView('outline');
     });
 
+  document.getElementById('viewOutline').addEventListener('dblclick',
+    function() {
+      PDFViewerApplication.outline.toggleOutlineTree();
+    });
+
   document.getElementById('viewAttachments').addEventListener('click',
     function() {
       PDFViewerApplication.switchSidebarView('attachments');