Added a delete button to list cells

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.
Merge branch 'master' into list-delete-button

Conflicts:
	src/watchers/ListCell.as
Fixed delete button showing on read-only lists
Merge pull request #222 from nathan/list-delete-button

List delete button