Fix memory leak in test

Change-Id: I026ce85a6e593d5eccecdf032ee2f0763a2bef87
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>
Warn when attaching Keys to an invalid item

Using Window { Keys.onPressed: ... } does not work because window is not
a QQuickItem. Warn at least.

Change-Id: Ibd472f7b551ff2089cbc39ba43da27e6f8e0e97f
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>