List delete button

Looks like this:

![screen shot 2014-05-28 at 18 14 58](https://cloud.githubusercontent.com/assets/1578238/3111795/b514134c-e6b5-11e3-89d5-9a320a5fbaf2.png)

This button can only be pressed when the cell is focused. There was an annoying bug in Scratch 1.4 where you could delete cells just by clicking on the empty space on the right side of the cell.