Add explicit and make public headers compile with -Wzero-as-null-pointer-constant

... or equivalent.

QtBase 5.6 headers already compile that way, so let the other
modules follow suit.

Added explicit where it was missing. This is not a source-
incompatible change, because code that breaks by this is
a bug. Let's not have this sitting around in an LTS.

Task-number: QTBUG-45291
Change-Id: I375361699615d0fc874e0b96a84543e233eff5f0
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>