Adds new panes to the window

def add(self, pane):
        """
        Adds new panes to the window
        """
        if isinstance(pane, list):
            initialised_panes = []
            for p in pane:
                initialised_panes.append(self.init_pane(p))
            self.panes.append(initialised_panes)
        else:    
            pane = self.init_pane(pane)
            self.panes.append(pane)