Changes `PDFOutlineView`/`PDFAttachmentView` to be initialized once, since we're always creating them, and refactor their `render` methods to instead pass in the `outline`/`attachments`.
For consistency with other "classes", the `PDFOutlineView`/`PDFAttachmentView` are renamed to `PDFOutlineViewer`/`PDFAttachmentViewer`.
Also, make sure that the outline/attachments are reset when the document is closed. Currently we keep the old ones around until the `getOutline`/`getAttachments` API calls are resolved for a new document.
