Modernize the Audio Devices example

Changed the style of member initialization.
Provided small cleanup.
Changed 0 to nullptr.
Changed to new signal/slot style if possible.

Task-number: QTBUG-60627
Change-Id: I995d62b8d3741c719565f5b203b1046e8701d238
Reviewed-by: Christian Stromme <christian.stromme@qt.io>