It is enough to just set the parent (and make the parent
call gtk_native_check_resize in size_allocate).
This commit removes the relative_to argument to the
constructors of GtkPopover and GtkPopoverMenu, and
updates all callers.
And let GtkPopover use it as its GtkAccessible implementation, this
accessible sets the POPUP_FOR relationship to the relative-to widget,
and keeps track of changes there.
https://bugzilla.gnome.org/show_bug.cgi?id=725864