Fix FormWindowEditor id.

Broke with recent refactoring and lead to drag&drop no longer working in
designer.

Change-Id: I33987b4fa8838ce1e009a665e9be80dbe30b45d6
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>