atkplug: Add atk_plug_new_child
As discussed on
https://mail.gnome.org/archives/gnome-accessibility-devel/2019-August/msg00001.html
in some cases such as gtk, one can't use the AtkPlug type directly as accessible object for the toplevel widget of the application, and one has to make AtkPlug a parent of the standard accessible object for that toplevel widget.
atk_plug_new_child() allows to achieve this easily by making the new AtkPlug a parent of the child passed as parameter.