Added metaDataChanged(key,value) signal to metadata controls.

It may be more efficient and convenient for application to use this
signal instead of metaDataChanged().

metaDataChanged() signal is emitted for compatibility,
backends should emit both signals.

Signal added QMetadataReader/WritterControl,
QMediaObject and QMediaRecorder.

Change-Id: I5ca34a40e64b22034e6fde8cd7f9735cc3f72c68
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>

src/multimedia/controls/qmetadatareadercontrol.cpp

@@ -126,6 +126,16 @@ QMetaDataReaderControl::~QMetaDataReaderControl()
     \fn void QMetaDataReaderControl::metaDataChanged()
 
     Signal the changes of meta-data.
+
+    If multiple meta-data elements are changed,
+    metaDataChanged(const QString &key, const QVariant &value) signal is emitted
+    for each of them with metaDataChanged() changed emitted once.
+*/
+
+/*!
+    \fn void QMetaDataReaderControl::metaDataChanged(const QString &key, const QVariant &value)
+
+    Signal the changes of one meta-data element \a value with the given \a key.
 */
 
 /*!

src/multimedia/controls/qmetadatareadercontrol.h

@@ -71,6 +71,7 @@ public:
 
 Q_SIGNALS:
     void metaDataChanged();
+    void metaDataChanged(const QString &key, const QVariant &value);
 
     void metaDataAvailableChanged(bool available);
 

src/multimedia/controls/qmetadatawritercontrol.cpp

@@ -139,6 +139,16 @@ QMetaDataWriterControl::~QMetaDataWriterControl()
     \fn void QMetaDataWriterControl::metaDataChanged()
 
     Signal the changes of meta-data.
+
+    If multiple meta-data elements are changed,
+    metaDataChanged(const QString &key, const QVariant &value) signal is emitted
+    for each of them with metaDataChanged() changed emitted once.
+*/
+
+/*!
+    \fn void QMetaDataWriterControl::metaDataChanged(const QString &key, const QVariant &value)
+
+    Signal the changes of one meta-data element \a value with the given \a key.
 */
 
 /*!

src/multimedia/controls/qmetadatawritercontrol.h

@@ -73,6 +73,7 @@ public:
 
 Q_SIGNALS:
     void metaDataChanged();
+    void metaDataChanged(const QString &key, const QVariant &value);
 
     void writableChanged(bool writable);
     void metaDataAvailableChanged(bool available);