wayland: Add support for v2 of xdg_foreign protocol
This updates the xdg-foreign protocol to the v2 version, spec here: xdg-foreign-unstable-v2.xml. The v1 version is still supported as a fallback. The corresponding patch for Mutter is here: mutter!1043 (closed). The corresponding patch in GTK3 is here: !1394. Here are the test programs I'm using to verify this: xdg-foreign-test, portal-test.