Refactor the "proxy action" behavior of Command to Utils::ProxyAction

A generic action that acts as a proxy for another (changeable) action,
like it is used for mostly all of the visible actions in Qt Creator
through the action manager.

src/plugins/helloworld/helloworldplugin.cpp

@@ -136,10 +136,6 @@ bool HelloWorldPlugin::initialize(const QStringList &arguments, QString *error_m
     Core::IMode *helloMode = new HelloMode;
     addAutoReleasedObject(helloMode);
 
-    // Add the Hello World action command to the mode manager (with 0 priority)
-    Core::ModeManager *modeManager = core->modeManager();
-    modeManager->addAction(command, 0);
-
     return true;
 }