QmlConsole: Fix infinite recursion bug

The queried v8 object sometimes returns a child item that
refers to itself triggering an infinite recursion.

Task-number: QTCREATORBUG-9747
Change-Id: I30dfbb834ebeb39c33a6050d4b4860b1b47f34d9
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
QmlConsole: Check for duplicate children

When constructing the console item tree, check for
duplicate entries.

Change-Id: Ibc5aa150287ef5e2ee4b691a71bb170308a1fde7
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>