[Xcb] xcb-proto-1.7 broke existing xcbgen API

[Alex Plotnick]

There's still fallout from this issue. Commit c5ff7fb fixed the build,
but re-opened a long-dead (well, burried, really) bug with the generated
request method for QueryTextExtents. This issue was first raised by
Aldo Cortesi in December 2009 in the mailing list thread "xpyb bub" [sic],
and a fix was commited as 7fb1a0d. But Julien's recent commit didn't 
preserve the logic of that patch exactly right, and so the generated
code once again referred to a non-existent "self.string_len" attribute.

My second patch, below, fixes the generated code. But I can't in good
conscience iterate over a field named "parent", so the first patch,
on which the second depends, continues the "parent" -> "parents" renaming 
in xcbgen. With these two patches, we now once again have a build that 
works *and* correctly generated code.

Again, just to be clear: patch 1/2 (Rename the ExprType "parent" attribute
to "parents") is a proto patch, and patch 2/2 (Fix length field handling,
again) is an xpyb patch. Such is life in a modular world.

Thanks,

        -- Alex