title: How to reposition Widgets from BoxLayout
body: <p>I'm using latest version of kivy: <code>v1.10.1.dev0, git-Unknown, 20171208</code>, python <code>v3.6.2</code>, mac os.</p>
  
  <p>I want to achive the following: when I update a value in settings reposition/redraw <code>BoxLayout</code> with the changed settings, in my case I'm updating <code>size_hint</code> value. Here is the work I have so far:</p>
  
  <p>my.py</p>
  
  <pre><code>from kivy.app import App
  
  
  class MyApp(App):
      def build_config(self, config):
          config.read('./my.ini')
  
      def build_settings(self, settings):
          settings.add_json_panel('Test', self.config, filename='my.json')
  
      def on_config_change(self, config, section, key, value):
          if section in ('layout',) and key in ('size_hint',):
              # NOT WORKING!!
              # print(self.root.canvas.clear())
              # self.root.canvas.ask_update()
              # self.root.canvas.draw()
  
              # NOT WORKING!!
              print(self.root)
              print(self.root.do_layout())
  
  if __name__ == "__main__":
      MyApp().run()
  </code></pre>
  
  <p>my.kv</p>
  
  <pre><code>#:import SettingsWithSidebar kivy.uix.settings.SettingsWithSidebar
  BoxLayout:
      RecycleView:
          size_hint: float(app.config.get('layout', 'size_hint')), 1
          Button:
              text: 'Open Settings'
              on_touch_down: app.settings_cls = SettingsWithSidebar; app.open_settings()
      RecycleView:
          size_hint: 1-float(app.config.get('layout', 'size_hint')), 1
          Button:
              text: 'Just a test'
  </code></pre>
  
  <p>my.ini</p>
  
  <pre><code>[layout]
  size_hint = 0.6
  </code></pre>
  
  <p>my.json</p>
  
  <pre><code>[
    {
          "type": "string",
          "title": "Size Hint",
          "desc": "Size Hint",
          "section": "layout",
          "key": "size_hint"
      }
  ]
  </code></pre>
  
  <p>So, how do I see the new layout updated with new value of <code>size_hint</code> when I close the settings window?</p>
  
  <p>Thx!</p>