ColorButton: Don’t destroy dialog @ ::delete-event

ColorButton: Don’t destroy dialog @ ::delete-event

Without specifically connecting ::delete-event to something, the dialog
will be destroyed when it is closed, for example by pressing Esc. This
meant that when dismissing it this way, unlike by pressing Cancel, any
custom palette would be lost when the dialog was next opened, and so on.

Resolve this by making ::delete-event just do GTK_RESPONSE_CANCEL, so
closing the dialog has the same effect as clicking its Cancel button.

https://bugzilla.gnome.org/show_bug.cgi?id=787444