Operator to duplicate the active Grease Pencil layer

TODO: this needs a proper "duplicate" icon, without the "ID" label

release/scripts/startup/bl_ui/properties_grease_pencil_common.py

@@ -361,6 +361,8 @@ class GreasePencilDataPanel():
 
         gpl = context.active_gpencil_layer
         if gpl:
+            sub.operator("gpencil.layer_duplicate", icon='COPY_ID', text="") # XXX: needs a dedicated icon
+
             col.separator()
 
             sub = col.column(align=True)