codegen/: Backport pygtk 2.6.2 codegen

codegen/: Backport pygtk 2.6.2 codegen

Original commit message from CVS:
* codegen/argtypes.py:
* codegen/codegen.py:
* codegen/definitions.py:
* codegen/defsparser.py:
* codegen/h2def.py:
* codegen/mergedefs.py:
* codegen/reversewrapper.py:
Backport pygtk 2.6.2 codegen
* gst/gst-types.defs:
Added fields for Buffer, Event, Message and Query
Added pre-roll for enum ElementStateReturn
* gst/gst.defs:
Added (define-method for gst_structure_nth_field_name()
* gst/gstelement.override:
Added override for gst_element_get_state
* gst/gstcaps.override:
re-enabled structure functions