[afp-renderer-developers] Changes to DTD resolution

[Joe S. <jo...@ex...>]

I've modified the DTDEntityResolver to handle the change in the DTD a
bit more gracefully:

* It now uses the public id of the doctype to decide what DTD to resolve
* It will throw an exception if a v1.0 DTD is specified, with an
appropriate exception message.

The major user visible effect of this change is that afp-fonts.xml
*must* have the following doctype definition:

<!DOCTYPE installed-fonts PUBLIC 
    "-//APACHE/DTD AFP Installed Font Definition DTD 1.1//EN"
    "http://org.apache.fop/installed-afp-fonts-1.1.dtd">

I'm in the middle of updating the docs to reflect this change.

Cheers,
Joe