Move file thumbnail element into the label tag

This moves the thumbnail into the "label".

Goal is to make it possible to add elements on the left of the thumbnail which is currently very hard at the moment. Instead of adding more CSS hackery, moving it into the label does the job.

Also it makes sense because we want clicking on the thumbnail to activate the checkbox, which is the role of the label...

Please review @nickvergessen @MorrisJobke 

Required for the "favorites" feature to be able to add a star icon on the left. (see https://github.com/owncloud/core/pull/12360 to see it in action)