Top/Bottom Node slab placement added (Stairs mod functionality)

~ Added function to auto-retireve list registered slabs, using slab group
~ Split glass slabs and normal slabs so these no longer combine - reason buggy
~ Added back the ability to place slab at top of node next too or at the bottom of node next too.
~ Insured slabs only place over other slabs,air and flora very explicit groups to try and increase compatability with other mods that might add small nodes and decrease potential conflicts
~ Added stairs mod "rotate and place" function to maintain identiucal functionality as stairs mod
~ Cleaned up placing behaviour so its more consistant with base game/what user might expect.
~ Added some error checking and messages to user