lazy-gtk-cat.scm: Fix broken exec and diff command handler.

* ui/gtk/lazy-gtk-cat.scm (lazycat-send-message): Fix broken exec
  and diff command handler.
  (lazycat-set-master-host): Fix malformed message.
* ui/gtk/output-view.scm (output-view-format)
  (output-view-format-list, output-view-format-diff): New methods.