Replace BaseMode convenience class by individual implementation.

Using the convienience class does not really save code and adds another
needless level in the hierarchy. This affects the three remaining BaseMode
users: Help, ProjectExplorer and HelloWorld.

src/plugins/helloworld/helloworldplugin.h

@@ -50,8 +50,6 @@ public:
 
 private slots:
     void sayHelloWorld();
-
-private:
 };
 
 } // namespace Internal