[SUG] Allow Zombies to interact with exits, buttons & pickups

[WillLem]

All 3 suggested behaviours implemented in Commits b9ae0fe4d (buttons), 37160f9f3 (exits) and 0f1154ba6 (pickups), along with dedicated sounds. The sounds can be updated at a later stage if we wish.

Let's give these a test run in 2.5 and see if we like it.

2.5 is shaping up to be "the Zombie update" more than anything. Since there are bugfixes in there as well, I might get it released soon and save the new skills for 2.6 instead.

[namida]

Yes; assigners and deassigners affect zombies. (So do neutralizers and deneutralizers, but a neutral zombie is functionally no different from a regular zombie anyway. And no, I don't think they should be, especially if it's a unique rule that doesn't apply to either a zombie or a neutral when not combined.)