Custom export or rendering commands

[maoschanz]

• in the prefs
  • all labels should change to be clearer
  • a workaround concerning the "show_all" and its unwanted effects has to be implemented
  • fix revealjs support (maybe it's just the URL?)
  • implement set_command_for_format
• in the export dialog
  • the entry will always be sensitive
  • the button and help label will always be visible