Prevent multiple bridges spawning at the same location.
Fix bogus pathability check.

This fixes units on pathable bridge tiles from
being killed on bridge death (they were failing
the check against themselves).
Rewrite Enter and related activities.

The unit will now path to the nearest adjacent cell, drag inside, do the inner activity, then (if still alive) drag back to its original
location.

This fixes:
 - Mismatch in logic between Enter and MoveAdjacentTo,
   which causes an infinite loop.
 - Building capturing failing from certain directions.
 - Being unable to enter buildings on unpathable tiles.
 - Units being stranded inside a building if the requirements
   for the inner order aren't met.
Add Selectable field to Selectable.

There are a bunch of bogus assumptions about
targetable actors being selectable. These aren't
easily fixed, so this add a Selectable field that
can be diabled for things we want to target, but
not select.
Replace UnitTraitOrderTargeter with TargetTypeOrderTargeter.

This also makes naval buildings untargetable for c4 and demo trucks, as they don't make much sense.
Add a ParentActorInit for FreeActor spawns.
Make bridges repairable.
Add BridgeHut and repair activities.
Add invisible repair zones to bridges.
Merge pull request #3063 from pchote/repairable-bridges

Repairable bridges, closes #2619