Using nullptr instead of Q_NULLPTR

[Ivan Čukić]

>> I prefer the first option, it's the way forward and if someone was using
>
> I'd say that requiring a newer gcc just like that would go against the
> nature of the KF5 project.

I don't really see why it is "against the nature of KF5". It would not
be the first time we require a higher compiler version than the one
required by the Qt version we require.

Qt 5.5 requires gcc 4.8 for linux and windows. So even they increment
the versions from time to time.

We can switch everything to Q_NULLPTR even without anyone complaining
about it, but we will have to switch back in a few releases.

Cheerio,
Ivan