scanner: Replace GInitiallyUnowned field sharing with generic solution

Remove GInitiallyUnowned special case in gdumpparser where fields are
copied from GObject. Add generic solution where anytime we have multiple
typedef structs, the fields become shared:

typedef struct _Foo Foo;
typedef struct _Foo Bar;
struct _Foo {...};

https://bugzilla.gnome.org/show_bug.cgi?id=581525