- fixed: The Hexen bridge must make its balls disappear when it gets destroyed.

Hexen did this with a call to A_BridgeRemove in Thing_Destroy which merely set a flag in the bridge object, which cannot be done safely in ZDoom because it's not guaranteed that the ball object calls A_BridgeOrbit and the garbage collector may delete the bridge actor before it can be checked so now the Bridge's Destroy method deletes all balls attached to the bridge object itself.
Merge branch 'maint'