Try harder to show help when webkit is missing or the help browser
won't start:

- offer to switch to the web browser when the help browser is
  present, but won't start for some reason
- in prefs, don't bother showing the help browser GUI when webkit
  is missing, otherwise show a warning label if the help browser
  is not installed even though webkit is there. Switch to the web
  browser in both cases (modified patch from lisanet)
- add OS X replacement for gtk_show_uri() in plug-in-web-browser
  (modified patch from lisanet)