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.

Cleaned up placement of * and & in parameters as a drive-by.

Added explicit where it was missing as a drive-by. 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: If81ed0c71393aee21d347f5ade4bf3fcc07cd82f
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>

src/multimedia/audio/qaudiodecoder.h

@@ -69,7 +69,7 @@ public:
         ServiceMissingError
     };
 
-    QAudioDecoder(QObject *parent = 0);
+    explicit QAudioDecoder(QObject *parent = Q_NULLPTR);
     ~QAudioDecoder();
 
     static QMultimedia::SupportEstimate hasSupport(const QString &mimeType, const QStringList& codecs = QStringList());

src/multimedia/audio/qaudioinput.h

@@ -56,8 +56,8 @@ class Q_MULTIMEDIA_EXPORT QAudioInput : public QObject
     Q_OBJECT
 
 public:
-    explicit QAudioInput(const QAudioFormat &format = QAudioFormat(), QObject *parent = 0);
-    explicit QAudioInput(const QAudioDeviceInfo &audioDeviceInfo, const QAudioFormat &format = QAudioFormat(), QObject *parent = 0);
+    explicit QAudioInput(const QAudioFormat &format = QAudioFormat(), QObject *parent = Q_NULLPTR);
+    explicit QAudioInput(const QAudioDeviceInfo &audioDeviceInfo, const QAudioFormat &format = QAudioFormat(), QObject *parent = Q_NULLPTR);
     ~QAudioInput();
 
     QAudioFormat format() const;

src/multimedia/audio/qaudiooutput.h

@@ -56,8 +56,8 @@ class Q_MULTIMEDIA_EXPORT QAudioOutput : public QObject
     Q_OBJECT
 
 public:
-    explicit QAudioOutput(const QAudioFormat &format = QAudioFormat(), QObject *parent = 0);
-    explicit QAudioOutput(const QAudioDeviceInfo &audioDeviceInfo, const QAudioFormat &format = QAudioFormat(), QObject *parent = 0);
+    explicit QAudioOutput(const QAudioFormat &format = QAudioFormat(), QObject *parent = Q_NULLPTR);
+    explicit QAudioOutput(const QAudioDeviceInfo &audioDeviceInfo, const QAudioFormat &format = QAudioFormat(), QObject *parent = Q_NULLPTR);
     ~QAudioOutput();
 
     QAudioFormat format() const;

src/multimedia/audio/qaudioprobe.h

@@ -47,7 +47,7 @@ class Q_MULTIMEDIA_EXPORT QAudioProbe : public QObject
 {
     Q_OBJECT
 public:
-    explicit QAudioProbe(QObject *parent = 0);
+    explicit QAudioProbe(QObject *parent = Q_NULLPTR);
     ~QAudioProbe();
 
     bool setSource(QMediaObject *source);

src/multimedia/audio/qaudiosystemplugin.h

@@ -72,7 +72,7 @@ class Q_MULTIMEDIA_EXPORT QAudioSystemPlugin : public QObject, public QAudioSyst
     Q_INTERFACES(QAudioSystemFactoryInterface)
 
 public:
-    QAudioSystemPlugin(QObject *parent = 0);
+    explicit QAudioSystemPlugin(QObject *parent = Q_NULLPTR);
     ~QAudioSystemPlugin();
 
     virtual QList<QByteArray> availableDevices(QAudio::Mode) const = 0;

src/multimedia/audio/qsound.h

@@ -52,7 +52,7 @@ public:
 
     static void play(const QString& filename);
 
-    explicit QSound(const QString& filename, QObject* parent = 0);
+    explicit QSound(const QString &filename, QObject *parent = Q_NULLPTR);
     ~QSound();
 
     int loops() const;

src/multimedia/audio/qsoundeffect.h

@@ -74,7 +74,7 @@ public:
         Error
     };
 
-    explicit QSoundEffect(QObject *parent = 0);
+    explicit QSoundEffect(QObject *parent = Q_NULLPTR);
     ~QSoundEffect();
 
     static QStringList supportedMimeTypes();