Doc: Fix warnings in StocQt Qt Quick demo

Fix a number of warnings from QML engine by adding
some boundary & sanity checks for properties. Also
address UI update issues by having XMLHTTPRequest
calls to trigger by a timer, instead of directly
binding them to multiple change notifier signals.

Task-number: QTBUG-33867
Change-Id: I48f573050b5dc3e3fe6e75bc423fe86ecc4a2469
Reviewed-by: Geir Vattekar <geir.vattekar@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>