Register self globally in C so it can be retrieved from iOS callbacks

Property definitions

ios :: app $ App :: register_globally
	# Register `self` globally in C so it can be retrieved from iOS callbacks
	private fun register_globally in "ObjC" `{
		App_incr_ref(self);
		app_nit_ios_app = self;
	`}
lib/ios/app.nit:116,2--120,3