Fixed bug 74: problems with @button if defined in myLeoSettings.leo: https://github.com/leo-editor/leo-editor/issues/74

- ScriptingController now uses *gnxs* (not vnodes!) to detect duplicate buttons.
- Use AtButtonCallback class for *all* buttons.
- QtIconBarClass.setCommandForButton always calls ScriptingController.build_rclick_tree.
- build_rclick_tree no longer assumes that command_p exists.

Leo build: 20141031092329