Getting app dev docs up to date for ownCloud 7

:construction: WIP

I'm working on documenting all the new stuff and some additional things that went (or will go) into oc7

To have a checkmark list of missing stuff:
* [x] API
* [x] Container
* [x] Background jobs
* [x] Controllers
* [x] CSS
* [x] Database
* [x] Filesystem
* [x] Javascript
* [x] L10n
* [x] Logging
* [x] Middleware
* [x] Testing
* [x] Users
* [x] Hooks
* [x] Templates
* [x] Tutorial
* [x] Settings