add a rudimentary intruder alerting system with tripwires

Currently wildlife will break into "animal pens" to murder any pets. While this is ok, there's currently little counterplay other than building a very large perimeter or fooling the ai with structures they can't path to. A rudimentary system that still requires player interaction of something like a tripwire and bell(rope+iron?) could provide a form of counterplay that still requires player interaction