Don't delete the QML engines from the profiler service destructor

Task-number: QTBUG-37382
Change-Id: Ib40d69813794e18cc7c7f05e10881b1b23a93b4e
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Doc: Qt QML: Document signals (not handlers) under \qmlsignal

Task-number: QTBUG-35846

Change-Id: I989617865a79bfbf1f1ffd3b105598025dde54c5
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>