QMediaPluginLoader: don't load the same plugin multiple times
QMediaPluginLoader: don't load the same plugin multiple times

Task-number: QTMOBILITY-1424
Reviewed-by: Jonas Rabbe

qtproject/qt-mobility,tmcguire/qt-mobility,KDE/android-qt-mobility,tmcguire/qt-mobility,kaltsi/qt-mobility,KDE/android-qt-mobility,tmcguire/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,enthought/qt-mobility,enthought/qt-mobility,enthought/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,KDE/android-qt-mobility,tmcguire/qt-mobility,kaltsi/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,KDE/android-qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility