title: wmctrl not executable while running sikulix script in headless mode
body: <p>I'm using Ubuntu 14.04 lts and sikulix 1.1.0.</p>
  
  <p>while running sikulix script in the ordinary way it's working great, but when i'm trying to use Xvfb <a href="https://stackoverflow.com/questions/26032706/how-to-get-sikuli-working-in-headless-mode">just like suggested here</a>, it's says that there is no wmctrl installed. and thats not true, this is the output in shell:</p>
  
  <pre><code>running SikuliX: -Xmx512M -Dfile.encoding=UTF-8 -Dsikuli.FromCommandLine
  -jar ./sikulix.jar -r ../sikuliscripts/script.sikuli
  [info] runcmd: lsb_release -i -r -s 
  [error] RunTimeIDE:  *** terminating: command 'wmctrl' is not executable
  please check the Availability!
  </code></pre>
  
  <p>thanks in advance!</p>