Rename namespace QtMultimedia::MetaData -> QMediaMetaData

Main code, examples, tests and docs updated. Method:
1. Remove unused forward-declarations, "class QMediaMetaData"
2. Mass find+replace "QtMultimedia::MetaData" -> "QMediaMetaData"
3. Un-nest from the QtMultimedia namespace in qtmedianamespace.h

For consistency (with minimal disruption), namespaces with a "Qt" prefix
will be renamed. Also, Qt guidelines don't include nested namespaces
(http://lists.qt-project.org/pipermail/development/2012-October/006756.html)

Part of the Header Consistency Project
(http://lists.qt-project.org/pipermail/development/2012-October/007570.html)

Change-Id: I40e59c1cf58c1792725e735e9285c51bc5f226b1
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

tests/auto/unit/qdeclarativeaudio/tst_qdeclarativeaudio.cpp

@@ -846,17 +846,17 @@ void tst_QDeclarativeAudio::metaData_data()
 
     QTest::newRow("title")
             << QByteArray("title")
-            << QtMultimedia::MetaData::Title
+            << QMediaMetaData::Title
             << QVariant(QString::fromLatin1("This is a title"));
 
     QTest::newRow("genre")
             << QByteArray("genre")
-            << QtMultimedia::MetaData::Genre
+            << QMediaMetaData::Genre
             << QVariant(QString::fromLatin1("rock"));
 
     QTest::newRow("trackNumber")
             << QByteArray("trackNumber")
-            << QtMultimedia::MetaData::TrackNumber
+            << QMediaMetaData::TrackNumber
             << QVariant(8);
 }