title: pygtk3 checking does signal exist
body: <p>how can I learn this before connecting anything.</p>
  
  <p>example</p>
  
  <pre><code>does_signal_exist(widget, signal):
      pass
  
  button = Gtk.Button()
  
  does_signal_exist(button, "clicked") # returns True
  does_signal_exist(button, "hello") # returns False
  does_signal_exist(button, "unboxed") # returns False
  does_signal_exist(button, "button-press-event") # returns True
  </code></pre>