Add GDBusActionGroup

GDBusActionGroup can be used as a client-side wrapper
for the an action group that has been exported on D-Bus.

gio/giotypes.h

@@ -48,6 +48,7 @@ typedef struct _GZlibCompressor               GZlibCompressor;
 typedef struct _GZlibDecompressor             GZlibDecompressor;
 
 typedef struct _GSimpleActionGroup            GSimpleActionGroup;
+typedef struct _GDBusActionGroup              GDBusActionGroup;
 typedef struct _GActionGroup                  GActionGroup;
 typedef struct _GSimpleAction                 GSimpleAction;
 typedef struct _GAction                       GAction;