GDBus: update padding

GDBus: update padding

Add lots of padding for public class structures. Notably, we seemed to
lack any padding whatsoever in the GDBusMessageClass struct (spotted
by Dan Winship). Also switch to using

 gpointer padding[N];

instead of

 void (*_g_reserved1) (void);
 ...
 void (*_g_reservedN) (void);

Signed-off-by: David Zeuthen <davidz@redhat.com>