use slideshow as image preview in files & public shares

This PR makes the slideshow available in the files app and in public file listings.
see https://github.com/owncloud/apps/issues/436

@jancborchardt @icewind1991 please review. I commented a line in the window.onhashchange function in gallery.js as it produced additional clicks on images which caused weird errors. The slideshow would not update the hash in the url when switching to the next image anyway. 

@karlitschek This obsoletes the files_imageviewer app, which is no longer necessary in future releases. It is replaced by a unified user experience for viewing files in files, gallery and public shared files.

@sualko This supersedes your PR https://github.com/owncloud/apps/pull/1111 Maybe you can test this, too and give me your feedback, too. If you think this is an improvement please also close your PR.