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>