Skip firing events when creating an MVC group instance

From GRIFFON-463 : There are times when creating multiple MVC groups where there's no need to trigger MVC events for example, a custom MVCGroupManager can potentially disable the event router for a time, then enable it after the group has been constructed.

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 firing, like this

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