wayland/xdg-foreign: Add support for protocol v2

This updates the xdg-foreign protocol to the v2 version, spec here: xdg-foreign-unstable-v2.xml. The corresponding patch for GTK is here: gtk!1394

For reference, the KDE support for this protocol is here: xdg_foreign_v2.cpp.

Support for the v1 protocol is still included has been moved to another file marked "legacy" simlar to the status of zxdg_shell_v6. Is this the right approach? Requesting feedback. If someone else wants to test this too that would be great. Here are the test programs I'm using: xdg-foreign-test, portal-test.