Steps to reproduce:
1. Open http://localhost:8888/web/viewer.html
2. Open the console, and run: `PDFViewerApplication.close();`
3. Note the warning: `[fluent] Missing translations in en-us: pdfjs-toggle-views-manager-button`
Given that we either use the `L10n` class directly or extend it via `GenericL10n`, it should no longer be necessary to keep the interface-definition.
This should help reduce the maintenance burden of the code, since you no longer need to remember to update separate code when touching the `L10n` class.
Update the styles and HTML to reflect the new views manager concept.
For now, nothing about split/merge functionality is implemented or visible.
The new styles for the outline, attachments, and layers will be added later.
The thumbnail view is now accessible with the keyboard.
