title: listen for a key when the application is not focused
body: <p>I've an application(C# 4.0-WPF), which is hidden and can be displayed by clicking on the systray icon or on an other frame I created(small frame which is docked left and topmost).</p>
  
  <p>My customer wants to add a new way to display the application: When pressing on a "F" key (e.g. <kbd>F9</kbd>).</p>
  
  <p>How do I know in my application if the user presses this key when the application is not the current window /or is not focused?</p>