Add Selectable field to Selectable.

There are a bunch of bogus assumptions that all
intractable units are selectable. These aren't
easily fixed, so add a Selectable field that can
be set false for things we want to target, but not
select.
Add a ParentActorInit for FreeActor spawns.
Make bridges repairable.
Add BridgeHut et all for repairing bridges.
Enable repairable bridges in C&C.