Improve robustness of QML plugin loading

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

Change-Id: I167c14ea612349f0ba0e9e29a129fb6c975504a0
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/imports/webview.cpp

@@ -45,7 +45,7 @@ QT_BEGIN_NAMESPACE
 class QWebViewModule : public QQmlExtensionPlugin
 {
     Q_OBJECT
-    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QQmlExtensionInterface/1.0")
+    Q_PLUGIN_METADATA(IID QQmlExtensionInterface_iid)
 public:
     void registerTypes(const char *uri)
     {