user-accounts: Use gtk_menu_popup_at_widget for user photo popup

gtk_menu_popup has been deprecated since version 3.22.

The use of gtk_menu_popup lately was causing gtk+ to have to guess
the right position to present the widget, causing the menu to
eventually appear in a wrong position under Wayland.

https://bugzilla.gnome.org/show_bug.cgi?id=780498