Add Bootstrap dropdown-header functionality.

This change adds Bootstrap Dropdown menu headers.
The PHP syntax is simply a menu item with no URL specified: ['label': 'This is a header']

Menu headers can be used as a nicer alternative to simple separators, especially with some extra styling. This code enhancement is necessary to enable the use of features like 'visible', which allows the creation of dynamic sectioned menus. Bad example but gets the point across: ['label': 'Administrators Only', 'visible': Yii::$app->user->checkAccess('manageUsers')] (and everything below would be admin-only menu items that don't show up for regular users)