Item creation

As a POC, implement a way to create a new item using the GUI.

In the end, we'll have to decide in the GUI how item (and property, and entity) creation will happen. Shall it be Hecate's responsibility, or should we let each "list" view do it as it want ?
However, specifying this behaviour is outside the scope of this issue. Thus, a kinda ugly/not thoroughly untested code is acceptable ... for now.