Since some browsers render `null` characters, and others don't, this patch adds a way to remove them to prevent display issues in the viewer UI.
Given that documents may contain very long outlines, I've added a utility function to avoid creating a lot of unnecessary `RegExp` objects.
To avoid any future issues, this utility function is used for both the outline and the attachments.
Fixes 6416.
- 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.
