don't generate an empty li for absent style menu in links area
	update css for Classic and Snappy to handle:
		dl lists
		links in package header and in declarations
		floating of links and info block in package and module headers

  make whole .caption be the target
  improve javascript for class toggling
  have plus/minus images come from .css, not img tags

  decided that --themes was silly - no one would do that, just use
    multiple --theme arguments
  made --theme a synonym for --css and -c
  made those arguments, if no file is found, look up the argument as the
    name of a built in theme

  all of this let's haddock be invoked with "--theme=classic" for example.

  The bulk of the change is threadnig the selected theme set through functions
  in Xhtml.hs so that the selected themes can be used when generating the page
  output. There isn't much going on in most of these changes, just passing it
  along. The real work is all done in Themes.hs.