Rename namespace QtMultimedia::MetaData -> QMediaMetaData

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>