There are a couple of structs and unions which uses inheritance in our stack that is currently quite problematic to introspection.

Non-complete list:
- GdkEvent (gdk)
- GMenuTreeItem (gmenu)
- GIBaseInfo (gobject-introspection itself)
- cairo_surface (cairo)

What they have in common is the following pattern:

typedef enum {
  BANANA_STRUCT = 1;
  APPLE_STRUCT = 2;
} StructKind;

StructKind base_struct_get_kind (BaseStruct *base);

BananaStruct {
  BaseStruct base;
  ...
};

AppleStruct {
  BaseStruct base;
  ...
};

To support this we'd need the following:
1) An annotation for each sub struct adding a relationg to the base struct, eg:
  BananaStruct: (child-of BaseStruct 1)
  AppleStruct: (child-of BaseStruct 2)
  The name of the struct and the type/kind value representation for that struct
2) The type getter, eg:
  BaseStruct: (child-getter base_struct_get_kind)

We'd need to extend the GIR and the typelib to support that as well.

Task list:
- implement/decide annotations
- update gir
- update typelib
- add tests
- document annotation