Split slide resources from the shared $resource.root

* Declare a $resource.slide XSLT param with a default value of
  $resource.root; if $resource.slide is not defined, this change has no
  user impact
* The slide resources are included via their locality to the
  $resource.root. This limits the flexibility of pulling in external
  slide resources, especially when using the xslt20-resource repository
Merge pull request #16 from bockel/slide-xslt-fixup

Split slide resources from the shared $resource.root