Cleanup and unify object manager methods

[ri0t]

There are too many pathways of logic in the objectmanager, which makes it unnecessarily complicated. With more clever handling of non-existing objects in the frontend, the code's complexity could be reduced by around 20%.

• [ ] Only create should generate a new object
• [ ] Only change should modify an existing object
• [x] Switch all methods to deferrables by adding request_id

Also makes testing easier, which C. Johnson would approve.

• [ ] (Update the tests!)

[ri0t]

Unifying the callers is necessary. Some more data (e.g. amount of search results) needs to be transported to some frontend components.