db: Fix unimplemented foreign key

Commit 59bcb7e09f6a172ddb48d451d076b0af8a477c8e activated the pragma
foreign_keys, but the database schema included a reference which is
still unimplemented. Therefore adding new windows failed and PalMa
was unusable.

Signed-off-by: Stefan Weil <sw@weilnetz.de>