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.
Make gtk_popover_new_from_model() return a GtkPopoverMenu,
rename it to gtk_popover_menu_new_from_model() and add
a relative_to argument to gtk_popover_menu_new().
Update all callers.
Remove all the old 2.x and 3.x version annotations.
GTK+ 4 is a new start, and from the perspective of a
GTK+ 4 developer all these APIs have been around since
the beginning.
