QmlProject: Always use qmlobserver for debugging projects

Also only warn about missing qmlobserver when actually trying to
debug a .qmlproject.

Reviewed-by: Christiaan Janssen

src/plugins/qmlprojectmanager/qmlprojectruncontrol.h

@@ -78,6 +78,7 @@ public:
 
 private:
     ProjectExplorer::RunControl *createDebugRunControl(QmlProjectRunConfiguration *runConfig);
+    static void showQmlObserverToolWarning();
 };
 
 } // namespace Internal