LibOverride: Add operator to convert a proxy object into an override.

In the end the process is surpringly simple, we only need to manually
convert the proxy itself into an override (which is trivial), and then
run common code with the default 'make override' operation.

Fix T81059: Add operator to convert proxies to library overrides.

release/scripts/startup/bl_ui/space_view3d.py

@@ -2191,6 +2191,8 @@ class VIEW3D_MT_object_relations(Menu):
 
         layout.operator("object.make_override_library", text="Make Library Override...")
 
+        layout.operator("object.convert_proxy_to_override")
+
         layout.operator("object.make_dupli_face")
 
         layout.separator()