[Xcb] generic events

[Thomas Hunger]

> Just as a warning: the integration of XGE into Xext uses a bit of
> "magic" and I'm not sure how to build a similar functionality into
> xcb.

I am a bit lost here. All the WireToEvent and EventToWire functions 
seem to do is search the extension for the event and call it's 
WireToEvent and EventToWire.

I believe that these wire conversion functions are not needed and not 
provided by XCB. Therefore we would not need that magic? 

Could we use an xml-description file for the query_version request?

Tom