StScrollView and StButton should support add_child()

Affected version

OS: 5.6.4-arch1-1

GNOME Shell: 3.36.1

Bug summary

I'm working on an extension for GNOME and noticed that StScrollView and StButton (and, possibly, other classes as well) does not support Clutter's add_child() method. They seem to still rely on deprecated add_actor().

Steps to reproduce

Create a StScrollView or StButton and try adding children with add_child().

What happened

Using add_child() throws no errors and added children are returned with parent.get_children(). However, none of them are visible. Setting child.visible to true has no effect.

What did you expect to happen

add_child() should produce the same output as add_actor(), i.e. added children should be visible.