wayland/xdg-foreign: Add support for xdg-foreign-v2

This replaces the v1 implementation, which is now renamed to
legacy-xdg-foreign. Both implementations use the same data structures
internally, so that protocol version mismatches between
the importer client and exporter client don't fail.

GNOME/mutter!1043
5 jobs for !1043 with xdg-foreign-v2 in 10 minutes and 29 seconds (queued for 1 second)
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