GPencil: Refactor Append operators

Now the operators work like other areas of Blender using the list of selected objects.

Also, the name has been changed to:

```Layers:
- Copy Layer to Selected
- Copy All Layers to Selected

Materials:
- Copy Material to Selected
- Copy All Materials to Selected```

Reviewed By: mendio, pablovazquez, pepeland

Differential Revision: https://developer.blender.org/D11216

release/scripts/startup/bl_ui/properties_data_gpencil.py

@@ -113,8 +113,8 @@ class GPENCIL_MT_layer_context_menu(Menu):
         layout.operator("gpencil.layer_merge", icon='SORT_ASC', text="Merge Down")
 
         layout.separator()
-        layout.menu("VIEW3D_MT_gpencil_append_active_layer")
-        layout.menu("VIEW3D_MT_gpencil_append_all_layers")
+        layout.operator("gpencil.layer_duplicate_object", text="Copy Layer to Selected").only_active=True
+        layout.operator("gpencil.layer_duplicate_object", text="Copy All Layers to Selected").only_active=False
 
 
 class DATA_PT_gpencil_layers(DataButtonsPanel, Panel):