Skip registering controllers as application event listeners

Sometimes you don't want controllers to be registered as application event listeners because their code never handles an event. This results in performance upgrades as controllers need not be notified.

This feature is generic enough to warrant inclusion in griffon core.
Proposal: specify a special key in the group's config section that marks this group for no event notification, like this

mvcGroups {
    // MVC Group for "foo"
    'foo' {
        model      = 'foo.FooModel'
        view       = 'foo.FooView'
        controller = 'foo.FooController'
        config {
            events {
                listener = false
            }
        }
    }
}