title: GMail like selection combobox implemented in GWT
body: <p>I'm interested in implementing something like the combobox used in GMail to easily select emails. The "default" selection of the combobox has a checkbox that can be clicked to select all emails, otherwise you can dropdown the box and choose another selection option.</p>
  
  <p><img src="https://i.stack.imgur.com/wkrCq.png" alt="enter image description here">
  How would you go about implementing this in GWT?</p>