Make GtkTreeView use GtkStyleContext

Expander animation has been replaced by implicit animations
from the style context.

Column headers are also properly themed, GtkContainer::get_path_for_child()
is implemented and the treeview resets column buttons style on columns
being reordered.

The default CSS has changed to theme treeviews sensibly.

gtk/gtktreeviewcolumn.c

@@ -1881,6 +1881,8 @@ gtk_tree_view_column_set_visible (GtkTreeViewColumn *tree_column,
   if (priv->visible)
     _gtk_tree_view_column_cell_set_dirty (tree_column, TRUE);
 
+  _gtk_tree_view_reset_header_styles (GTK_TREE_VIEW (priv->tree_view));
+
   gtk_tree_view_column_update_button (tree_column);
   g_object_notify (G_OBJECT (tree_column), "visible");
 }