Improved the header frame so that it can be reset many times.

Also added an error message frame, which is packed at the bottom of the pane.

wizpym.py

@@ -82,6 +82,13 @@ class Pane(gtk.VBox):
         gtk.VBox.__init__(self, False, 5)
 
         self._frame = frame
+
+        self._header = gtk.VBox()
+        self.pack_start(self._header, False)
+
+        self._error = gtk.VBox()
+        self.pack_end(self._error, False)
+
         self._subject = None
         self._switch = None
         self._successors = {}
@@ -111,11 +118,31 @@ class Pane(gtk.VBox):
         explaining the current step, calling this method will ensure
         such panes all present the same header style.
         """
-        help_label = gtk.Label(msg)
-        help_label.set_justify(gtk.JUSTIFY_FILL)
-        help_label.set_line_wrap(True)
-        self.pack_start(help_label, False, False, 10)
-        self.pack_start(gtk.HSeparator(), False, False)
+        self._header.foreach(lambda w: self._header.remove(w))
+        if msg:
+            help_label = gtk.Label(msg)
+            help_label.set_justify(gtk.JUSTIFY_FILL)
+            help_label.set_line_wrap(True)
+            self._header.pack_start(help_label, False, False, 10)
+            self._header.pack_start(gtk.HSeparator(), False)
+            self._header.show_all()
+
+    def set_error(self, msg):
+        """Add a help header to the pane with the given message.
+
+        Since it is quite common that panes begin with a little guide
+        explaining the current step, calling this method will ensure
+        such panes all present the same header style.
+        """
+        self._error.foreach(lambda w: self._error.remove(w))
+        if msg:
+            error_label = gtk.Label()
+            error_label.set_markup(
+                '<span foreground="red" weight="bold">' + msg + '</span>')
+            error_label.set_justify(gtk.JUSTIFY_FILL)
+            error_label.set_line_wrap(True)
+            self._error.pack_start(error_label, False, False, 10)
+            self._error.show_all()
 
     def switch(self, klass):
         """Update the switch using the given class.