title: "Select all/Deselect all" in SAPUI5 SelectDialog
body: <p>I have a SAPUI5 SelectDialog. It contains multiple entries. Right now, UI5 provides property to clear all the selected values using <code>showClearButton="true"</code>
  <a href="https://i.stack.imgur.com/mQRzq.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/mQRzq.png" alt="enter image description here"></a></p>
  
  <p><strong>What I want to achieve?</strong>
  I want to implement similar functionality, to "Select all" entries in the dialog. I know sap.m.tableSelectDialog provides this, but how can I do this in SelectDialog? Possible solutions can include </p>
  
  <ol>
  <li>Check box at top, which says "Select/Deselect all"</li>
  <li>Button in footer of SelectDialog, which says "Select/Deselect all". <a href="https://i.stack.imgur.com/FL2IP.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FL2IP.png" alt="enter image description here"></a></li>
  </ol>
  
  <p>Any suggestions are welcomed.  </p>