QmlProject: Disable debug action if qmlobserver cannot be built

QmlObserver right now requires 4.7.1 minimum. There's no gain in
telling the user that he needs qmlobserver to debug, if it can't be
build with 4.7.0 anyway.

Reviewed-by: Christiaan Janssen

src/plugins/qmlprojectmanager/qmlprojectrunconfiguration.h

@@ -74,6 +74,7 @@ public:
     QString observerPath() const;
     QStringList viewerArguments() const;
     QString workingDirectory() const;
+    Qt4ProjectManager::QtVersion *qtVersion() const;
 
     // RunConfiguration
     virtual QWidget *createConfigurationWidget();