title: FragmentNavigator add Fragment instead of replace for transparent background
body: <p>When using the <a href="https://developer.android.com/guide/navigation" rel="noreferrer">Navigation</a> library from AndroidX, I'd like to display a Fragment with a semi-transparent background on top of the old destination. So basically, instead of the <a href="https://developer.android.com/reference/androidx/navigation/fragment/FragmentNavigator" rel="noreferrer">FragmentNavigator</a> doing <code>ft.replace(mContainerId, frag)</code> inside its <a href="https://developer.android.com/reference/androidx/navigation/fragment/FragmentNavigator.html#navigate(androidx.navigation.fragment.FragmentNavigator.Destination,%20android.os.Bundle,%20androidx.navigation.NavOptions,%20androidx.navigation.Navigator.Extras)" rel="noreferrer">navigate()</a> method, having a <code>ft.add(mContainerId, frag)</code> instead.</p>
  
  <p>I've tried subclassing <code>FragmentNavigator</code>, but the API is very strongly limited (lots of private variables and package protected methods), thus not being able to properly override the <code>navigate()</code> method in order to make that change.</p>
  
  <p>So, what would be the correct method to achieve support for Fragments with transparent background using the Navigation library?</p>