These generate basic .c and .h files containing the GDBusInterfaceInfo
for a D-Bus introspection XML file, but no other code (no skeletons,
proxies, GObjects, etc.).

This is useful for projects who want to describe their D-Bus interfaces
using introspection XML, but who wish to implement the interfaces
manually (for various reasons, typically because the skeletons generated
by gdbus-codegen are too simplistic and limiting). Previously, these
projects would have had to write the GDBusInterfaceInfo manually, which
is painstaking and error-prone.

The new --interface-info-[body|header] options are very similar to
--[body|header], but mutually exclusive with them.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=795304