By moving popup layout emission to the layout phase, the current
GdkPopup::poup-layout-changed signal has no value on its own as it'd be
ignored by GtkPopover.
Make the Wayland backend communicate the popup layout changes via the
common signal; but leave the rest intact until other backends catch up.
This is not quite right, and only temporary, since
it makes GDK_IS_POPUP (surface) true for every surface.
Eventually, the implementation will be moved to the
backends.
