Simon McVittie authored May 13, 2024 and Philip Withnall committed May 16, 2024

Most D-Bus interfaces are domain-specific, but these interfaces from the
D-Bus Specification are intended to be commonly used in any context for
which they are found to be appropriate.

Most of these use `gdbusprivate.h`. One exception is that
`gio/tests/gdbus-example-*` redefine the constants locally: due to these
files' dual role as part of the unit tests and as sample code, it seems
desirable to ensure that they can still be compiled outside GLib.

Signed-off-by: Simon McVittie <smcv@collabora.com>