title: First TreeViewColumn is not rendering
body: <p>I have an window defined in a gtk-builder file made with Glade. It contains a <code>gtk.Window</code>, with a child <code>gtk.TreeView</code> with 2 <code>gtk.TreeViewColumn</code>s:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;interface&gt;
      &lt;requires lib="gtk+" version="2.20"/&gt;
      &lt;!-- interface-naming-policy project-wide --&gt;
      &lt;object class="GtkWindow" id="bookmarks_window"&gt;
          &lt;property name="can_focus"&gt;False&lt;/property&gt;
          &lt;property name="events"&gt;GDK_KEY_PRESS_MASK | GDK_STRUCTURE_MASK&lt;/property&gt;
          &lt;property name="title" translatable="yes"&gt;Bookmarks&lt;/property&gt;
          &lt;property name="modal"&gt;True&lt;/property&gt;
          &lt;property name="default_width"&gt;350&lt;/property&gt;
          &lt;property name="default_height"&gt;200&lt;/property&gt;
          &lt;property name="destroy_with_parent"&gt;True&lt;/property&gt;
          &lt;property name="type_hint"&gt;menu&lt;/property&gt;
          &lt;property name="skip_taskbar_hint"&gt;True&lt;/property&gt;
          &lt;child&gt;
            &lt;object class="GtkScrolledWindow" id="bookmarks_scroll"&gt;
              &lt;property name="visible"&gt;True&lt;/property&gt;
              &lt;property name="can_focus"&gt;True&lt;/property&gt;
              &lt;property name="hscrollbar_policy"&gt;never&lt;/property&gt;
              &lt;property name="vscrollbar_policy"&gt;automatic&lt;/property&gt;
              &lt;child&gt;
                &lt;object class="GtkTreeView" id="bookmarks_tree"&gt;
                  &lt;property name="visible"&gt;True&lt;/property&gt;
                  &lt;property name="can_focus"&gt;True&lt;/property&gt;
                  &lt;property name="events"&gt;GDK_BUTTON_PRESS_MASK | GDK_KEY_PRESS_MASK | GDK_STRUCTURE_MASK&lt;/property&gt;
                  &lt;property name="headers_visible"&gt;False&lt;/property&gt;
                  &lt;property name="headers_clickable"&gt;False&lt;/property&gt;
                  &lt;property name="rules_hint"&gt;True&lt;/property&gt;
                  &lt;property name="fixed_height_mode"&gt;True&lt;/property&gt;
                  &lt;property name="show_expanders"&gt;False&lt;/property&gt;
                  &lt;property name="enable_grid_lines"&gt;vertical&lt;/property&gt;
                  &lt;child&gt;
                    &lt;object class="GtkTreeViewColumn" id="line_column"&gt;
                      &lt;property name="sizing"&gt;fixed&lt;/property&gt;
                      &lt;property name="title" translatable="yes"&gt;Line&lt;/property&gt;
                    &lt;/object&gt;
                  &lt;/child&gt;
                  &lt;child&gt;
                    &lt;object class="GtkTreeViewColumn" id="text_column"&gt;
                      &lt;property name="sizing"&gt;fixed&lt;/property&gt;
                      &lt;property name="title" translatable="yes"&gt;Text&lt;/property&gt;
                    &lt;/object&gt;
                  &lt;/child&gt;
                &lt;/object&gt;
              &lt;/child&gt;
            &lt;/object&gt;
          &lt;/child&gt;
      &lt;/object&gt;
  &lt;/interface&gt;
  </code></pre>
  
  <p>I'm creating the window doing:</p>
  
  <pre><code>def show_window(parent_window):
      builder = gtk.Builder()
      builder.add_from_file("mywindow.ui")
  
      window = builder.get_object('bookmarks_window')
      window.set_transient_for(parent_window)
  
      tree = builder.get_object('bookmarks_tree')
      tree.set_search_column(1)
  
      store = gtk.TreeStore(int, str)
      for i in xrange(1, 6):
          store.append((i, "Line {}".format(i)))
      tree.set_model(store)
  
      line_renderer = gtk.CellRendererText()
      line_column = builder.get_object('line_column')
      line_column.pack_start(line_renderer)
      line_column.set_attributes(line_renderer, text=0)
  
      text_renderer = gtk.CellRendererText()
      text_column = builder.get_object('text_column')
      text_column.pack_start(text_renderer)
      text_column.set_attributes(text_renderer, text=1)
  
      window.show_all()
  </code></pre>
  
  <p>This will only show the "Text" column:</p>
  
  <pre><code>+---------------+
  | Line 1        |
  | Line 2        |
  | Line 3        |
  | Line 4        |
  | Line 5        |
  +---------------+
  </code></pre>
  
  <p>If I change the order of the <code>gtk.TreeViewColumn</code>s in the interface file from:</p>
  
  <pre><code>&lt;child&gt;
    &lt;object class="GtkTreeViewColumn" id="line_column"&gt;...&lt;/object&gt;
  &lt;/child&gt;
  &lt;child&gt;
    &lt;object class="GtkTreeViewColumn" id="text_column"&gt;...&lt;/object&gt;
  &lt;/child&gt;
  </code></pre>
  
  <p>to:</p>
  
  <pre><code>&lt;child&gt;
    &lt;object class="GtkTreeViewColumn" id="text_column"&gt;...&lt;/object&gt;
  &lt;/child&gt;
  &lt;child&gt;
    &lt;object class="GtkTreeViewColumn" id="line_column"&gt;...&lt;/object&gt;
  &lt;/child&gt;
  </code></pre>
  
  <p>Only the "Line" column will display:</p>
  
  <pre><code>+---------------+
  | 1             |
  | 2             |
  | 3             |
  | 4             |
  | 5             |
  +---------------+
  </code></pre>
  
  <p>But I want it to look like:</p>
  
  <pre><code>+---+-----------+
  | 1 | Line 1    |
  | 2 | Line 2    |
  | 3 | Line 3    |
  | 4 | Line 4    |
  | 5 | Line 5    |
  +---+-----------+
  </code></pre>
  
  <p>Changing the order of the <code>gtk.TreeViewColumn</code>s in the interface file is changing what column is displayed, which doesn't make sense to me. It's always hiding the first column. Does anyone know what I am missing or overlooking? Any help would be appreciated.</p>