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 |
+--------------------+------------------------------------+