Improve robustness of QML plugin loading

Use a variable that expands to the latest plugin version instead of
hard-coding the string.

Change-Id: I3bdbee161e4a3b59127dc4f00a0c9ce9386b67c2
Reviewed-by: Yoann Lopes <yoann.lopes@qt.io>

src/imports/audioengine/audioengine.cpp

@@ -63,7 +63,7 @@ QT_BEGIN_NAMESPACE
 class QAudioEngineDeclarativeModule : public QQmlExtensionPlugin
 {
     Q_OBJECT
-    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QQmlExtensionInterface/1.0")
+    Q_PLUGIN_METADATA(IID QQmlExtensionInterface_iid)
 
 public:
     QAudioEngineDeclarativeModule(QObject *parent = 0) : QQmlExtensionPlugin(parent) { initResources(); }

src/imports/multimedia/multimedia.cpp

@@ -81,7 +81,7 @@ static QObject *multimedia_global_object(QQmlEngine *qmlEngine, QJSEngine *jsEng
 class QMultimediaDeclarativeModule : public QQmlExtensionPlugin
 {
     Q_OBJECT
-    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QQmlExtensionInterface/1.0")
+    Q_PLUGIN_METADATA(IID QQmlExtensionInterface_iid)
 
 public:
     QMultimediaDeclarativeModule(QObject *parent = 0) : QQmlExtensionPlugin(parent) { initResources(); }