Let gtk-doc grok GSEAL

[Tim Janik <timj imendio com>]

Hi Stefan,

as mentioned during the last IRC meeting:
  http://live.gnome.org/GTK%2B/Meetings?action=AttachFile&do=view&target=20080722.txt

It'd be nice if gtk-doc
understood the GSEAL macros. I guess the handling would be best, if:
- GSEAL(field); is treated like /*<private>*/ field;
- optionally, gtk-doc could ignore GSEAL(), so we can still generate
  docs for older gtk versions where some unsealed fields are treated
  as public read-only API.

Could you please look into supporting this?

---
ciaoTJ