title: How to force a widget (dropdown box) value programmatically?
body: <p>I'm setting in place a visualization which data are driven by selections made through bokeh widgets (mainly dropdowns).
  The dropdowns are linked (i.e: depending on dropdown 1 choice, dropdown2 will be filled with certain data, etc...)</p>
  <p>I have 2 needs which tend to be the same:
  I know I can get the value of a choice by using dropdown1.value (in an on_change callback) so:</p>
  <ul>
  <li><p>How can I <strong>set</strong> the value of a dropdown (and force it to display this choice) ?</p>
  </li>
  <li><p>How can I reset all dropdowns to the default startup position?</p>
  <p>Thanks for the help!</p>
  </li>
  </ul>