See property_change for an explanation of the meaning of format.

Depending on the value of format, the property value(s) can be:

+--------------------+------------------------------------+
|      format        |                value               |
+--------------------+------------------------------------+
| Gtk2::Gdk::CHARS   | a string                           |
| Gtk2::Gdk::USHORTS | one or more unsigned short numbers |
| Gtk2::Gdk::ULONGS  | one or more unsigned long numbers  |
+--------------------+------------------------------------+