menu: refactoring (close #363)

  * get rid of `menu__group_has-title` modifier
  * menu-item_checked: use `background` instead of `::before`
menu-item: theme inherit (bh templates)
menu: TODOs + examples
menu: links must inherit `disabled` mod
Merge pull request #364 from bem/issues/363@v2

menu: refactoring (close #363)