You are not allowed to add components that belong to other aggregate
(including aggregates themselves). Warn in this case instead of locking
up.

Task-number: QTCREATORBUG-4926
Change-Id: I4908fb9019efbc2fa3b7c3c57e08cc1d7f8f3e2c
Reviewed-on: http://codereview.qt.nokia.com/679


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Bill King <bill.king@nokia.com>