new file -- clipboard.c

Copy layer -> layer address
on paste, layer_copy_func in Interface. returns new layer with new data, etc.

-- items --

if selected thing something to be copied call
layer_copy_item_func
then
layer_paste_item_func
If same layer is selected.

The only problem is determining if selected item can be copied.
No, wait, piece of cake. Hand back sublayer_type to the layer in a new interface func

gboolean layer_sublayer_is_copyable_func ( sublayer_type )

to determine if copyable