When using the `--header --body` compatibility mode, we need to emit
things we generally define in the header, such as the aliases for
standard marshallers, and aliases for deprecated tokens.
This fixes dbus-binding-tool, which is using `--header --body` and
deprecated tokens.
See: https://bugs.freedesktop.org/show_bug.cgi?id=101799
We're in the process or rewriting other tools in Python to reduce the
number of dependencies of GLib.
Additionally, making glib-genmarshal a Python script reduces the
complexity when cross-compiling, as we don't need a native build to
generate the marshallers.
https://bugzilla.gnome.org/show_bug.cgi?id=784528
