title: NixOS list all possible configuration options
body: <p>I started using NixOS a few days ago. It seems like there is a configuration option for everything, e.g. <code>hardware.opengl.driSupport32Bit</code> or <code>system.copySystemConfiguration</code>.</p>
  
  <p>But the question is: can I see all options which are available? To give a few examples:</p>
  
  <ul>
  <li>How should I know what desktopManagers are available other than the "kde4", "kde5" and "gnome3"?</li>
  <li>What synaptics options can I use, other than <code>accelFactor</code> and <code>twoFingerScroll</code>?</li>
  </ul>