nav-aggregate: customize title/lang/css, nav-create-page-list: set base

 * added option to set the base URI that paths are resolved against in nav-create-page-list, like already available for nav-create-toc
 * added options for setting the title, language and stylesheet when aggregating the navs
   * when title is specified, that title is used
   * otherwise, if language is specified, the string "Table of Contents" in the appropriate language (automatically translated using http://nicetranslator.com/ - "Navigation Document" didn't translate as well)
   * otherwise (or if language string is not defined), use the english translation: "Table of Contents"
   * the link to the css is only added if the css href is specified
   * the xml:lang and lang attributes are only added if the language option is specified
   * all three options are optional
can't use p:value-available in the static context of p:use-when
p:template doesn't take sequence as source