Implement reply-reading as multimethod

Imeplemnt reply-reading as multimethod to make adding new read functions
easier, without having to insert new entry into case-statement. This
will also make dispatching to manually written read functions possible.
Include manually written functions

Include manually written functions at the end of the generated files.
Insert header comment in generated files

Insert header comment in generated files, as a reminder not to modify
such files.
Regenerate xproto
Update README

Update README, as ConfigureWindow and QueryTextExtents have been
implemented.