Adaptative select in chisel mode

Adapt the selection for the mode of the chisel tool.
To see what we will delete.
It works on BlockStair and BlockDetailed.

![capture du 2014-11-21 07 34 47](https://cloud.githubusercontent.com/assets/401965/5139975/7e303334-7165-11e4-90b4-d4ab2fe77469.png)