libgimp*: lots of doc fixes and new docs for structs and enums

(cherry picked from commit aca5f806c4)

libgimpwidgets/gimpbusybox.h

@@ -55,6 +55,10 @@ struct _GimpBusyBoxClass
   void (* _gimp_reserved2) (void);
   void (* _gimp_reserved3) (void);
   void (* _gimp_reserved4) (void);
+  void (* _gimp_reserved5) (void);
+  void (* _gimp_reserved6) (void);
+  void (* _gimp_reserved7) (void);
+  void (* _gimp_reserved8) (void);
 };