[ALOY-1061] Alloy: Require a specific view via XML

In JS you can get a specific controller view (including its children of course) by passing its ID to getView() like this:

var view = Alloy.createController('foo').getView('bar');

I'd like to do the same in XML:

<Alloy>
  <Require src="foo" view="header">
  <View id="someOtherContent">..</View>
  <Require src="foo" view="footer">
</Alloy>

Please note that the implementation should only create the foo controller *once*, so that both the header and footer view share the same controller. Otherwise it wouldn't make much sense since the actual benefit/use-case is that these 2 views can interact with/on each other while providing views at 2 or more different locations in te parent controller/view.