stable6 icons.css backports

As talked about in https://github.com/owncloud/core/pull/7452

This is an improvement to the icons.css file we introduced. It’s a clean backport but nevertheless I’m doing it as pull request so everyone can check cc @karlitschek @DeepDiver1975 @owncloud/designers @tanghus

icons.css is an addition and does not break existing stuff and it was backported to enable app devs to use the same code in ownCloud 6 and master. Without these backports, that use is moot.

Please review.