title: Kivy: accessing a method on a different class
body: <p>Let's pretend I am building a tic-tac-toe game (becouse it's pretty similar as a structure)
  I want the result to be shown in a popup, with a new game button, and I want this popup to let me access settings (with another button) and change them, always staying within the popup, then leave and finally close it and start a new game.</p>
  
  <p>I wish i could keep things ordered and therefore have a separate popup class where i can build my custom popup.</p>
  
  <p>I have the newgame method and reset method as method of my game-grid class, as obvious. 
  Methods for changing settings are, on the other hand, on a custom settings class</p>
  
  <p>While designing the popup class how can I bind it's buttons (e.g new game) to methods that are contained on a completly different class? I've looked on some kv examples and they usually use root.blabla.method to acces a method that is in a different position of the same tree (in the .kv file) but here the methods I am trying to reach are out of the tree!</p>
  
  <p>I'll try to put some example code to make it more clear</p>
  
  <pre><code>class Settings():
  
      def changeSettings(self):
          ....
  
  class GmeGrid(GridLayout):
  
      def newGame(self):
          ....
  
      def reset(self):
          ...
  
  class customPopup(Popup):
  
      pass
  </code></pre>
  
  <p>Then, on a .kv file I wish I could bind some popup's buttons to the newGame and change settings methods</p>
  
  <p>The problem here is that I should bind buttons on the popop class to the mothods of completly different class and I don't know how to to that (on the .kv file especially)</p>