Using nullptr instead of Q_NULLPTR

[Ivan Čukić]

This is from the "Officially supported platforms" page at
http://doc.qt.io/QtSupportedPlatforms/

Qt 5.5:
Windows * - MinGW 4.9, MinGW 4.8 (apart from MSVC)
Linux - 32/64bitgcc 4.8.1, gcc 4.9.1

I thought that was official enough.

Cheers,
Ivan