Better Slabs Mod

BUG FIXES:

- fixed slabs not dropping anything

- rewrote placement modes:

    - removed fill mode

    - place are now placed as you expect (no, literally, they will be placed the way you actually think the would)

- revamped crafting recipes

    - you can convert vanilla slabs to this mod's slabs, and vice-versa, by placing them in the crafting table

    - snow slabs are crafted using 4 snow layers

    - current issue: vanilla stone slabs and mod stone slabs aren't correlated (the conversion between them makes no sense) and as a result stone
      slabs from this mod can't be crafted

- added End Brick Slab

- some optimizations and code tweaks