title: Control DiscolsurePanel collapsing by another button instead of header text
body: <p>I went through GWT Showcase and I found disclosure panel.
  I want to control disclosure panel collapsing and expanding by clicking on one button.
  In certain situations button can be disabled, In that case I want to enable/disable button.</p>
  
  <p>Please refer this for more details:</p>
  
  <p><img src="https://i.stack.imgur.com/FLwR2.png" alt="enter image description here"></p>
  
  <p>Please suggest, How to do this?</p>