Moves a layer, creates a new layer, or deletes a layer.

Arguments:

old | The is the layer number to act upon. Allowed values are: c = Currently selected layer. -1 = Create a new layer. number = An existing layer number.
new | Specifies where to move the layer to. Allowed values are: -1 = Deletes the layer. up = Moves the layer up (TODO: is this supported?) down = Moves the layer down (TODO: is this supported?) step+ = Moves the layer towards the end of its group's list. step- = Moves the layer towards the beginning of its group's list. c = Creates a new layer. grp gid = move layer into group; target group ID is the 3rd argument