[ALOY-656] Must have index.xml even if no UI is needed for the index controller

Even when no UI is needed for the index controller, Alloy requires that an index.xml be present, even if it is empty:

<Alloy/>

The reason is that Alloy and other tools (TiStudio for example) rely on the presence of index.xml as an indication that you are in fact building an Alloy app. Unfortunately, there are situation where the index controller needs know UI, only logic. Take for example the situation if you want to open a login screen or a home screen depending on queried data (either locally remotely). You need no UI to complete this query and determine whether the login controller or home controller should be opened, but the index.xml is required just the same. We need to remove this requirement and identify a more concrete method of determining if you are actually building an Alloy app.