Use 0 instead of Q_NULLPTR in public headers.

Otherwise Q_NULLPTR appears in documentation, which is inconsistent.

Change-Id: I0f4b07b25c6012d502938aa452e68c75d910f954
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Merge remote-tracking branch 'origin/5.3' into dev

Conflicts:
	src/imports/qmlwebsockets/qqmlwebsocket.h

Change-Id: Ib883a07eb1784743adca7a0c38fd156800354f4b