Refactor: Add Croogo.beforeSetupAdminData and use global event manager

In some cases, it might be required to setup admin environment
before the action is run.  This event will be triggered at the
end of CroogoAppController::beforeFilter() when core setup has
just been completed.

To automate setup for these type of events, plugins can add
Config/admin.php that will be `require`d automatically.

The existing Croogo.setupAdminData event can then be used as
currently implemented for cases where data are only available
just before a view is rendered.
Refactor: Automatically load plugin menu in ExtensionEventHandler

It does not feel right to create an event handler just to add menu.
So, do this for all plugins automatically.  This is done via the
ExtensionEventHandler::onSetupAdminData() method.

Plugins just need to add their own Config/admin_menu.php.