QmlDesigner: Show warning about invalid kit

Task-number: QTCREATORBUG-12578
Change-Id: I4308518bc883d4665d09bb9340557f89f30dc371
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>