util/uuid: replace with util/token, remove libuuid

Use 128-bit hexadecimal string tokens generated with /dev/urandom
instead of UUIDs for xdg-foreign handles, removing the libuuid
dependency. Update readme and CI. Closes #2830.

build: remove xdg-foreign feature

With no external dependencies required, there's no reason not to always
build it. Remove WLR_HAS_XDG_FOREIGN as well.

include/util/token.h

@@ -0,0 +1,9 @@
+#ifndef UTIL_TOKEN_H
+#define UTIL_TOKEN_H
+
+#include <stdbool.h>
+
+#define TOKEN_STRLEN 33
+bool generate_token(char out[static TOKEN_STRLEN]);
+
+#endif

include/util/uuid.h

@@ -1,8 +0,0 @@
-#ifndef UTIL_UUID_H
-#define UTIL_UUID_H
-
-#include <stdbool.h>
-
-bool generate_uuid(char out[static 37]);
-
-#endif