* widget-color-combo.c (apply_current_color): New
function. Contains meat of previous version of preview_clicked.
(preview_clicked): Make behaviour depend on instant_apply toggle.
(color_combo_construct): Initialize instant_apply.
(color_combo_set_instant_apply): New function. Turn instant apply
behaviour on or off.
(color_combo_get_color, color_combo_set_color,
color_combo_set_color_to_default): Add preconditions.

* widget-color-combo.h (struct _ColorCombo): Remove unused
'trigger' member. Add 'instant_apply' member.