title: Bokeh Python conditional CheckboxGroup
body: <p>Is there an easy way to create a conditional <code>CheckboxGroup</code> within the <code>Bokeh</code>-framework? Conditional in the sense that when another <code>Bokeh</code>-widget is triggered, the <code>CheckboxGroup</code> is populated based on the value within the triggering widget.</p>
  
  <p>So for instance, if I am having a dropdown-menu in form of a <code>Select</code>-widget with options <code>[a, b]</code>, and a separate Python-dict pointing at these "new" options <code>{'a' : ['aa','bb','cc'], 'b' : ['dd','ee','ff']}</code> - how do I create a checkbox based on the specific choice?</p>
  
  <p>Pseudo code:</p>
  
  <pre><code>options_dict = {'a' : ['aa','bb','cc'], 'b' : ['dd','ee','ff']}
  dropdown = Select(title="a dropdown menu:", options = ['a', 'b'], value='a')
  
  # let all sub-options be the default option
  checkbox = CheckboxGroup(labels=['aa','bb','cc','dd','ee','ff'], active=['aa','bb','cc','dd','ee','ff'])
  
  # assume we have some function that is updating and fetching the value of the 'dropdown'
  
  def select_something():
      selected_attribute = dropdown.value()
      return selected_attribute
  
  def update():
      selected_attribute = select_something()
      # update the CheckBox contents - obviously Pseudo-code
      checkbox.labels = options_dict[selected_attribute]
      # and have all of them "pre-checked"
      checkbox.active = options_dict[selected_attribute]
  
  dropdown.on_change('value', lambda attr, old, new: update())
  </code></pre>
  
  <p>Thanks</p>