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.
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| 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
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