Toggle app navigation not only on mobile, but on desktop as well [WIP!]

Work in progress (!), only a concept for now, but please check if this is basically ok @karlitschek @PVince81

Several reasons for this:
* We now have a sidebar in Files, so there’s two sidebars taking space (most other apps have a sidebar already).
* The app navigation is arguably not needed all the time.
* If you have many apps installed, scrolling on the app navigation is clunky and gets ugly fast (too thin)
* If you don’t have many apps installed, it looks like a waste of space (like for the Enterprise Edition)
* We give apps the whole width, which alleviates a lot of issues we had in the past with CSS and sizing

I also ported the animation fixed by @raghunayyar in #8278 over here.

Still TODO:
* [ ] is it clear enough that the apps are behind the logo / caret? Maybe we need a different icon
* [ ] or show the app menu on the first start of ownCloud
* [ ] maybe as additional indicator, show the app name between the logo and the caret. So it looks like »ownCloud Files«