Added Gates to Red Alert

This adds gates https://github.com/OpenRA/OpenRA/issues/3196 to the game so that can be manually opened, script triggered or run automatized. It is done in a simple fashion reusing the existing https://github.com/OpenRA/OpenRA/wiki/Traits#transforms. They don't interact with path-finding.

You can use http://resource.openra.net/maps/1711 for testing purposes.
That should be solved by actors either having a trait that toggles gate interactivity or a flag on something preexisting.

Others may want different locomotion types to trigger gates.