title: Multi-color text in one Gtk.Label
body: <p>I'm using a css file for theming my interface. So I have currently this:</p>
  
  <pre><code>    style_file = Gio.File.new_for_path('interface.css')
      style_provider = Gtk.CssProvider()
      style_provider.load_from_file(style_file)
  
      Gtk.StyleContext.add_provider_for_screen(
          Gdk.Screen.get_default(),
          style_provider,
          Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION
      )
  
      self.label = Gtk.Label()
      self.label.set_name('foobar_label')
      self.label.set_markup("&lt;span color='blue'&gt;foo &lt;/span&gt;"
                            "&lt;span color='green'&gt;{} &lt;/span&gt;"
                            "&lt;span color='red'&gt;"
                            "bar&lt;/span&gt;".format(baz))
  </code></pre>
  
  <p>I want to get rid of set_markup call and use the css for style it. I really must create three separated labels just for color it?</p>