dm4-files: implement a file topic renderer which allows users to select an existing file topic ...

[Malte]

The webclient editor does not allow for reference file topics as childs of composites.

When editing a "composite" topic, e.g. a "Webpage Element" which can also consists of just a name and file topic (more specifically has a "dm4.files.file" object aggregated, one to one) the user cannot select/reference any existing file topic from one of her (potentially workspace-) repos while editing a "Webpage Element". Currently, where the file topic selector should be nothing is rendered.

In the future this should alos be possible with "many" relationships but at the moment one to one would be sufficient to support.