Chisel [mode slab] on the Block Stair and Block Details

Changing the chisel tool, by making changes on the slab mode.

The idea is to make the tool more universal.
Removing impossibilities.
Today we can not use the slab tool on BlockStair, or on BlockDetailed.
With this proposal we can.
After your imagination do the rest.

![slab](https://cloud.githubusercontent.com/assets/401965/5036990/738c9db4-6b84-11e4-8914-88473b877f47.png)

![slabmod](https://cloud.githubusercontent.com/assets/401965/5038113/610191fa-6b8d-11e4-87ba-fb42e675266d.gif)