Enhancement: Added a toolbar to pdf-viewer app

We've added a toolbar to the pdf-viewer app,
where the user can see the name of the opened pdf file and also close the app.

https://github.com/owncloud/web/pull/7201
https://github.com/owncloud/web/pull/7207
https://github.com/owncloud/web/issues/7198
https://github.com/owncloud/web/issues/7205
