Add native support for uninterruptible activities

This pattern occurs a few times throughout the code, so it makes sense to bake it into the Activity class itself.

OpenRA.Mods.Common/Scripting/CallLuaFunc.cs

@@ -43,10 +43,13 @@ namespace OpenRA.Mods.Common.Activities
 			return NextActivity;
 		}
 
-		public override void Cancel(Actor self)
+		public override bool Cancel(Actor self)
 		{
+			if (!base.Cancel(self))
+				return false;
+
 			Dispose();
-			base.Cancel(self);
+			return true;
 		}
 
 		public void Dispose()