Update Bluetooth examples for high-DPI displays and nullptr

This change sets the Qt::AA_EnableHighDpiScaling application attribute.
It also uses nullptr to initialize pointers and adds a trivial
optimization to the low energy scanner example.

Change-Id: Ia923573ad78108a2a6a72be4c0948ea97a7ec094
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>