Check to prevent a child view element to have the same ID as the first root (by default)

By default, the first root view takes it `id` from the name of the view. This pull request adds a check to prevent a child view form having been set to the same `id`. If the first root view has been manually set to an `id`, the check does not happen.

Why this? Well... never make the same mistake twice:
* https://twitter.com/fokkezb/status/330260692575862785
* https://twitter.com/FokkeZB/status/333164024290226176