Bugfix: Filename hovers over the image in the preview app

We've fixed a bug where the filename hovers over the image content in the preview app and the bottom toolbar is not
accurate centered.
Therefore we have introduced a new top bar, where the filename will be shown
and the download and the close button will be displayed.

https://github.com/owncloud/web/pull/7216
https://github.com/owncloud/web/pull/7359
https://github.com/owncloud/web/issues/6300
