Wire up List Height option

This wires up the "List Height" PreValue - it was there but didn't look to be hooked up yet (apologies if I missed something!)

Preview when List Height is set to 200px
![xptl-200px](https://cloud.githubusercontent.com/assets/1396376/2869652/e9f015b4-d280-11e3-84cd-9458bf22906d.png)

The scrollbar should remove it self (`overflow-y: auto`) automatically when it's not needed:
![xptl-200px-2](https://cloud.githubusercontent.com/assets/1396376/2869653/ef2ca31c-d280-11e3-9a79-274a30c11bd3.png)

When left blank or 0, the list will be fluid as is now.