title: JList render invisible "selection marker"
body: <p>The JList supports multiple selection when you holding the control key: Press Ctrl+Up/Down to move some invisible marker (nimbus LAF). If you no press space, the element gets selected.</p>
  
  <p>Example: Your JList has three elements, the first one is selected. You know press Ctrl + Down, Ctrl + Down and then Space. The last element is now selected. </p>
  
  <p>The question is: How can I render the invisible marker I move with Ctrl+up/down? </p>
  
  <p>For example the windows file explorer renders this marker with a dotted border  and I like to render something similar. The thing is that with Ctrl + Up/Down you don't change the selection but you change the element which would be selected/deselected if you press Space.</p>