Refactoring ColorLabelComboBoxModel and ColorLabelWidget

• Refactoring implementing NotificationListener interface from ColorLabelComboBoxModel to ColorLabelWidget::ColorLabelWidgetPrivate.
• Fixing incorrect index calculations.
• ColorLabelWidgetPrivate implements functions selectColor(QString) and selectColor(QColor) to simplify selecting a color, optionally setting the user color in case the preferences do not include the color.
• ColorLabelWidget::clear() now actually 'clears' the color: resetting to NoColor, setting user color also to NoColor.
• Wrapper function ColorLabelWidget::currentColor() to retrieve the currently selected color.