Moved the interfaces into the Constants class on a trial basis (we may still

Moved the interfaces into the Constants class on a trial basis (we may still
choose to remove them in favor of strictly using Constants.)  Two interfaces
(span and position) were removed because of conflicts with other class names.

The interface generation was moved into Constants.xsl, which is run manually
via the XsltToJava ant task and its output is then checked in.  As a result,
no more autogeneration of these interfaces at build time will be done.

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@197213 13f79535-47bb-0310-9956-ffa450edef68