[feature] Nested submenus for top menu in protostar

# Description
This enables protostar to display nav-pills menus with two or more levels of submenus. 
# Testing instructions
1. Create a menu with more than two levels in backend or use one of the ones provided with sample data. In my pictures, I used the "About Joomla" menu from sample data.
2. Create a menu module
  * Type: Menu
  * Position: position-1
  * Select Menu: Your menu from step 1
  * Start level: 1
  * End level: all
  * Show Sub-menu Items: Yes
3. Save / publish the module.
4. Set Protostar as your frontend template.
5. Hover over the menu items in frontend. The first level of sub-items will be shown, but nothing beyond that.
6. Apply the PR
7. Now, all levels should be accessible in the dropdown menu.
JoomlaCode item: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32092
Old PR: https://github.com/joomla/joomla-cms/pull/2073