Arachnids on normal enemy force

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#Changes

• Arachnids are now on the same force as vanilla biters, and won't attack each other.
• A bit faster
• Green blood particles on hit and on dying
• Spawning only on desert areas (as default option)
• Only one nest for arachnid biters and spitters
• Added a new biter class for very late games: The Leviathan
• Added bosses units (only for use by another mods)
• Added proper icons

#Thanks / Credits -for Aotenshin for the great adition on the mod portal with the original Arachnids mod (https://mods.factorio.com/mod/Arachnids) -and to MnHebi for making those biters as a new biter force, not replacing vanilla enemies.