backport_master_gallery_to_stable5

only a few calls from the public API are not available in OC5 but there are few changes in the gallery I would like to see in the next OC5 release as well, which this PR achieves:
- use Pictures as the image preview in files.
- don't autoplay slideshow
- fixes and cleanups to the gallery / slideshow /thumbnails js & css code

@DeepDiver1975 this PR should make porting back https://github.com/owncloud/apps/pull/1476 easier 
@DeepDiver1975 don't know about the l10n folder ... just copied the files from master
@karlitschek please review