It'll be used to generate ID in the same way in other places later. https://bugzilla.gnome.org/show_bug.cgi?id=784199
This commits adds support for exporting xdg_surface handles via xdg_exporter and importing them via xdg_importer. This bumps the required wayland-protocols version to 1.6. https://bugzilla.gnome.org/show_bug.cgi?id=769786