Make QTemporaryDir file name more random

qrand() returns a number between 0 and RAND_MAX, which is only
guaranteed to be bigger than 32767. Dividing it repeatedly means
that the last bytes are always 0.

[ChangeLog][QtCore][QTemporaryDir] Fixed bug in QTemporaryDir name generator that dramatically
reduced randomness of the name.

Change-Id: I90613a652e6384296aed827e2714fe63cd8797ee
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>