Fix syntax error in Japanese translation
Remove remenants of mob code

Since minetest has no mobs within the core anymore, I suppose these
settings and code should go. Any mod that uses `minetest.setting_getbool`
will work with no problem since the default return value is `false`.
Remove m_ext_ptr in GUIFormSpecMenu, replaced by refcount mechanism
Add NodeResolver and clean up node name -> content ID resolution system