Borrow fixed orientation handling from NME

so that on iOS orientation can be configured before the stage is created.  See also https://github.com/haxenme/nme/commit/b7698f25d30d45d13c780f33567973772cca01f6.

Stage.shouldRotateInterface() cannot be used to indicate initial orientation preference on iOS, because it is not called unless the stage has been instantiated, and UIStageViewController.supportedInterfaceOrientations() is called before the stage is created.