Feature backend integration after integration commitment

It is the backend integration for some of the fields for the backend master (commit: 825c6197b84b50e80ef3d8763177e6c51ad2c1ef).

The models are not fully integrated (especially contacts, properties, attachments and customfields won't be saved). But as it is important to have the main branch a bit closer to our current work, we want to have those changes merged.