add DisplayItemUtility to centralize static routines

[BryanWilhite]

the first purpose for a DisplayItemUtility is to centralize how grouping (and sorting of groups) happens for MenuDisplayItemModel

[BryanWilhite]

one purpose for Selectable.map is to select grouping data that will set the groupId and groupDisplayText properties of Groupable

after a grouping is selected then it is always sorted, ascending or descending

[BryanWilhite]

BTW: added Groupable.isCollapsed, learning from IndexGroup

[BryanWilhite]

i am pretty sure that this is as close to e=mc**2 as i am going to get:

/**
 * return the data to display a flat list of items
 * as nested groups
 */
public static displayInGroups(items: MenuDisplayItemModel[], groupId?: string | number): MenuDisplayItemModel[] {
    DisplayItemUtility.setGrouping(items, groupId);
    const groups = DisplayItemUtility.group(items, groupId);
    return DisplayItemUtility.nestIntoGroups(groups);
}