Fix placement of the .active class for dropdown items & remove dropdown when item has no visible children

[IanDelMar]

Fixes #431 (misplaced .active class). Fixes dropdown markup on items without children (reported in https://github.com/wp-bootstrap/wp-bootstrap-navwalker/issues/517#issuecomment-909436992).

Changes proposed in this Pull Request:

• Defines 3 new boolean variables for better readability: $is_dropdown, $is_dropdown_item, $is_active. They indicate whether a menu item is a dropdown, a dropdown item and whether the menu item is active.
• Removes the .active class from the li-element and add it to the a-element if the item is a dropdown item. (https://getbootstrap.com/docs/4.6/components/dropdowns/#active)
• Removes the dropdown markup if a menu item has children but display is restricted by $args-depth.

Proposed changelog entry for your changes:

Fix misplaced .active class Remove dropdown markup on items without visible children