Feature: Optiongroups

[JoryHogeveen]

Adds a simple method for creation option groups for custom column classes.

For multiselect:
Selected columns will still be shown as first and outside of the group to be able to easily find active columns and deselect them if needed.

Format:

// Array key is the group label.
array(
	'One' => array(
		'one-half',
		'one-third',
		'one-fourth',
		'one-fifth',
		'one-sixth',
		'one-seventh',
		'one-eighth',
	),
	'Two' => array(
		'two-thirds',
		'two-fourths',
		'two-fifths',
		'two-sixths',
		'two-sevenths',
		'two-eighths',
	),
	// Etc.
);