title: Android, change fragment textview from activity
body: <p>I've one activity with 3 fragments,
  - top fragment has a textview,
  - center fragment has main content
  - bottom fragment has a tabwidget</p>
  
  <p>at the start I want my top fragment to display a "text", but when I click on a tab and the center fragment shows me another fragment (this is working fine), I want to display another text in my top fragment. </p>
  
  <p>how can I do this?</p>
  
  <p>thanks,
  Pedro Simão</p>