QmlProject: Prevent renaming .qmlproject files

That doesn't work, it doesn't look to hard to fix that, but the use case
for it is rather small anyway.

Task-number: QTCREATORBUG-10934
Change-Id: Idcb1fab96a67330e998590b70760b3c97e4cb780
Reviewed-by: Kai Koehne <kai.koehne@digia.com>