Questionnaire command and control

Nearly all of the pieces are in place. Note that I used a lot of binding expressions in the Wizard component so far, but I haven't shown either the top-level view model or how the wizard deals with user interactions. I always like to keep controllers slim and the view controller here is no exception:

// packages/wizard/src/view/WizardController.js
Ext.define('Wizard.view.wizard.WizardController', {
    extend: 'Ext.app.ViewController',
    alias: 'controller.wizard',
    listen: {
        component: {
            '#next': { click: 'onNextClick' },
            '#prev': { click: 'onPrevClick' },
            '#restart': { click: 'onRestartClick' },
            '#finish': { click: 'onFinishClick' },
            'wizard-progress button': { click: 'onStepClick' }
        }
    },

    onNextClick: function() {
        var current = this.getViewModel().get('currentPosition');
        this.getViewModel().set('currentPosition', current + 1);
    },

    onPrevClick...