icons: automatically show delete hover instead of using explicit class

Fix the need for app developers to manually add the hover state.

Please review @tanghus @owncloud/designers 

Also, we should probably backport all the icons.css changes. This, and https://github.com/owncloud/core/pull/7382 (by @jbtbnl), and https://github.com/owncloud/core/commits/master/core/css/icons.css Ok @karlitschek @DeepDiver1975?