title: sap.ui.commons.Panel Collapse / Expanded event
body: <p>is it possible to add an event to SAPUI5 <code>sap.ui.commons.Panel's</code> minimize and maximize? What i need is to add a different style when the Panel's collapse- or expand-Button is clicked.</p>
  
  <p>If the user clicks on the minus-Symbol i have to add a styleclass like <code>pnl.addStyleClass("XYZ");</code> when the user clicks on "maximize" a different styleclass should be added to the panel <code>pnl.addStyleClass("ZYFDEFD");</code></p>
  
  <p>if i overwrite the <code>.setCollapse-</code> or <code>.setExpanded-function</code> the panel doesn't collapse or expand anymore, so something else has to be found.</p>