Checks for id references in RelativeLayouts that are not defined elsewhere

The @+id/ syntax refers to an existing id, or creates a new one if it has not already been defined elsewhere. However, this means that if you have a typo in your reference, or if the referred view no longer exists, you do not get a warning since the id will be created on demand. This check catches errors where you have renamed an id without updating all of the references to it.