Zombies no longer hydrophobic + swimmer zombies

This PR is for issue #2503.

- Zombies (and other non-electronic things with MF_NO_BREATHE) will now chase you into deep water (but won't wander into it idly)
- Monsters that can travel through water will dive (or sink, depending on how you look at it), you can't see them if you're not in the water yourself
- Swimmer zombies! They're just regular zombies with MF_SWIMS, and they drop some of the WATER_FRIENDLY items. Some of them even have wetsuits. They spawn randomly, but groups of them can be found near pools.