scanner: Use descriptions in entries

entry may have a description according to the DTD. This is used in
some protocols including xdg-shell.

Fixes the code comment on an enum declaration using the description of
the last enum that had one, adds the descriptions to the comments on
enumerators, and avoids leaking the previously missing descriptions.

Fixes #208

Signed-off-by: James Legg <lankyleggy@gmail.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>