Add support to varying monster spawn

It was implemented to tfs master on otland/forgottenserver@6a8a663

and on rl tibia on 10.70

ability to declare more than one type of monster that will be respawned after death:

	<spawn centerx="103" centery="120" centerz="7" radius="5">
		<monsters x="-1" y="1" z="7" spawntime="180">
			<monster name="Rabbit" chance="70" />
			<monster name="Skunk" chance="30" />
		</monsters>
	</spawn>

This can be of course mixed with single monster node:

	<spawn centerx="103" centery="120" centerz="7" radius="5">
		<monster name="Troll" x="4" y="-1" z="7" spawntime="60" />
		<monsters x="-1" y="1" z="7" spawntime="180">
			<monster name="Rabbit" chance="70" />
			<monster name="Skunk" chance="30" />
		</monsters>
	</spawn>