This replaces the old GtkColorSelectionDialog, and has the convenience
advantage that the actual chooser (with all the 'set colour', 'get
colour' methods) and the containing dialog box are now the same object
implementing multiple interfaces, so I don't keep having to call 'get
me the underlying chooser for this dialog' accessors. Also you now
hook into both the OK and Cancel buttons (and all other response
codes) at the same time with a single event.
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