Z-order is for SafeWindow is not set in QNX Screen adaptation

The window Z-order setting is implemented using the environment variable, but the function that sets the property is not called.
The functionality of the environmental variable should be documented as well.

The problem can be reproduced in the NXP iMX8QM hardware by running QtCluster and Telltales examples. Telltales are not visible because QtCluster is in the foreground.