Copied xsl/extensions up a level to create a top-level

Copied xsl/extensions up a level to create a top-level
xsl-java module. Rationale is that we need to do a
separate docbook-xsl-java release in order to get the
XSL Java extensions packaged for Debian, and having it
as a top-level module allows the release build and
packaging to be done using the common build files in
releasetools/ and so makes it a lot easier to set up
and maintain the build and do the packaging and upload, etc.
After OK, would be best to next remove the
xsl/extensions directory from version control and
instead update the docbook-xsl build to simply copy
over the built jars from the xsl-java dir.