Allow modifying wxComboBox from its CLOSEUP handler with wxGTK2

Doing this resulted in GTK errors about invalid iterators, so postpone the
generation of the CLOSEUP event for slightly later to allow changing the
combobox contents from it with GTK+ 2 (this is not necessary with GTK+ 3).

Also add a demon of a dynamic combobox, creating and destroying its items on
the fly, to the widgets sample.

Closes #17223.

interface/wx/combobox.h

@@ -67,8 +67,7 @@
            Process a @c wxEVT_COMBOBOX_CLOSEUP event, which is generated
            when the list box of the combo box disappears (closes up). This
            event is only generated for the same platforms as
-           @c wxEVT_COMBOBOX_DROPDOWN above. Also note that only wxMSW and
-           wxOSX/Cocoa support adding or deleting items in this event.
+           @c wxEVT_COMBOBOX_DROPDOWN above.
     @endEventTable
 
     @library{wxcore}