Before c4a2234a28
menu models could use markup for items and the markup would
be parsed, but this was not intended behavior.
This commit adds official support for using markup
for menu items via the `use-markup` property.
Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/4306
Add a way to add children at certain places in
the generated menu for both GtkPopoverMenu and
GtkPopoverMenuBar.
New apis:
gtk_popover_menu_add_child
gtk_popover_menu_remove_child
gtk_popover_menu_bar_add_child
gtk_popover_menu_bar_remove_child
Fixes: #3260