title: How to add buttons with icons in python toga
body: <p>I am creating a gui app for GTK with python-toga and was wondering if it was possible to create a <code>toga.Button()</code> with an icon. I've tried using <code>toga.Command()</code> to create a command which has an option to add an icon but adding the command to a box doesn't work.</p>
  
  <p>This is the part of my code that doesn't work:</p>
  
  <pre><code>search_box = toga.Box()
  search_box.direction = 'row'
  search_box.style.padding = 10
  search_box.style.flex = 1
  
  search_label = toga.Label('Search')
  search_label.style.padding = (10, 10, 10, 0)
  
  search_input = toga.TextInput(placeholder='Search')
  search_input.style.flex = 1
  search_input.style.color = "#FFFFFF"
  
  search_command = toga.Command(action=self.test, label='Search') # self.test is a function that just prints user input taken from search_input
  search_command.tooltip = 'Search For The Item'
  search_command.icon = 'icons/search.png'
  
  search_box.add(search_label)
  search_box.add(search_input)
  search_box.add(search_command) # This line errors out
  </code></pre>
  
  <p>I get the following error when I run the above code:</p>
  
  <pre><code>root@host:~/c0balt# briefcase dev
  
  [c0balt] Starting in dev mode...
  [GTK+] Not implemented: Widget.set_hidden()
  [GTK+] Not implemented: Widget.set_hidden()
  [GTK+] Not implemented: Widget.set_hidden()
  [GTK+] Not implemented: TextInput.set_font()
  [GTK+] Not implemented: Widget.set_hidden()
  [GTK+] Not implemented: TextInput.set_font()
  [GTK+] Not implemented: TextInput.set_alignment()
  [GTK+] Not implemented: TextInput.set_font()
  [GTK+] Not implemented: TextInput.set_font()
  [GTK+] Not implemented: TextInput.set_font()
  Traceback (most recent call last):
    File "/usr/local/lib/python3.7/dist-packages/toga_gtk/app.py", line 91, in gtk_startup
      self.interface.startup()
    File "/root/c0balt/src/c0balt/app.py", line 41, in startup
      search_box.add(search_command)
    File "/usr/local/lib/python3.7/dist-packages/toga/widgets/base.py", line 66, in add
      super().add(child)
    File "/usr/local/lib/python3.7/dist-packages/travertino/node.py", line 80, in add
      set_root(child, self.root)
    File "/usr/local/lib/python3.7/dist-packages/travertino/node.py", line 6, in set_root
      for child in node.children:
  AttributeError: 'Command' object has no attribute 'children'
  
  </code></pre>
  
  <h1>Tl;dr</h1>
  
  <p>How to add buttons in python-toga that have an icon instead of just plain text?</p>