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

Changing the chisel tool, by making changes on the slab mode.
So as to allow the use on BlockStair and on BlockDetailed.

![slab](https://cloud.githubusercontent.com/assets/401965/5036990/738c9db4-6b84-11e4-8914-88473b877f47.png)