Implement Mob zones

You can now define zones with mobs. The mobs will spawn at random in the
zone.
Mobs can only move inside a zone, and they can go from one zone to
another when trying to attack a player.
You can have as many mobs per zone as you want, or no mobs if you want.

At the moments mobs can't move, attack, or take damages. They just spawn
at a random position inside the zone (if my math is right).
Remove the dragon at Zecoras

We'll add real dragons in their own mobzones later.
Add some mobs in the Gem Mines

Two lantern monsters and a diamond dog