Moved param.xsl build up, out of html subdir. Rationale is too

Moved param.xsl build up, out of html subdir. Rationale is too
allow better integration with DocBook XSL stylesheets build (if
we decide to move slides under xsl in source repository), and to
enable inclusion of potential fo params and params for other
slides output formats (svg, etc.) -- without needing to deal with
additional complexity of having a separate params.xsl file for
each slides output format.