PaperUI: Ensure proper caching of thing-types during initial rendering

[htreu]

With #3731 a huge refactoring re-introduced the problem of #3010. The code from #3731 will request the same thing-type several times for each item respectively. The JavaScript repository API needs to change in a way that callbacks will be executed in sync with the calling method OR (which would be even better) the whole process should be wrapped in a promise based processing.