Cleanly build with --warn-fatal.  Implementation:

* Liberally apply (skip) where the API is clearly C only, e.g. uses
  XLib.  The theming code and MutterPlugin are skipped too.
* Add missing (transfer) and (element-type) annotations

For a few functions that had a comment, I turned it into gtk-doc, but
I didn't (with a few exceptions) try to write new documentation in
this pass.