QtSupport: Add a method to retrieve the QML path

We check that qmake property in many different places.

Change-Id: Ifd5efe4ad2831385493bd3afe8538929578e8fb4
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/android/androiddebugsupport.cpp

@@ -138,7 +138,7 @@ void AndroidDebugSupport::start()
         setQmlServer(m_runner->qmlServer());
         //TODO: Not sure if these are the right paths.
         if (qtVersion)
-            addSearchDirectory(qtVersion->qmakeProperty("QT_INSTALL_QML"));
+            addSearchDirectory(qtVersion->qmlPath().toString());
     }
 
     // FIXME: Move signal to base class and generalize handling.