Describe ComboBox.set_active and use it in example

Closes #178

Author: cydanil

examples/combobox_example.py

@@ -62,6 +62,7 @@ def __init__(self):
         for currency in currencies:
             currency_combo.append_text(currency)
 
+        currency_combo.set_active(0)
         vbox.pack_start(currency_combo, False, False, 0)
 
         self.add(vbox)

source/combobox.txt

@@ -13,6 +13,9 @@ If the combo box contains a large number of items, it may be better to display
 them in a grid rather than a list. This can be done by calling
 :meth:`Gtk.ComboBox.set_wrap_width`.
 
+A default value can be set by calling :meth:`Gtk.ComboBox.set_active` with the index
+of the desired value.
+
 The :class:`Gtk.ComboBox` widget usually
 restricts the user to the available choices, but it can optionally have an
 :class:`Gtk.Entry`, allowing the user to enter arbitrary text if none of the