Direct edit on edge labels is not triggered by F2 or direct typing

[pcdavid]

Steps to reproduce:

1. Create a new project from the "Studio" template.
2. In the diagram, select the edge (named "entity1s") between Root and Entity1.
3. Click on the "direct edit" tool in the edge's palette: the label's direct edition is started (OK)
4. Hit F2 => nothing happens.
5. Start typing some text => nothing happens.

On nodes, F2 and direct typing start the label edition process.