Feature Request: Add Ability for Chairs to be used on slabs

[ToffeeMax]

As title says, but asked for a few places.

• Chairs current only uses stairs
• Allow pigs /saddle on slabs to add more variety

[Bloo-dev]

This has been discussed within the team briefly, and the consesus seemed slightly against it.

Personally, I think this wouldn't add too much and add issues such as not being able to tell the facing of a slab-chair. Hence, I am against this.