Enhancement: wait for all required data

Before this we rendered the ui no matter if every required data already is loaded or not.
For example the current users language from the ocis settings service.
One potential problem was the flickering in the ui or that the default language was shown before it switches to the settings language of current user.
Instead we now show a loading screen and wait for everything that is required before rendering anything else.

https://github.com/owncloud/ocis/issues/884
https://github.com/owncloud/ocis/issues/1043