dropdown: get rid of wrapping root div node

[aristov]

Now dropdown has wrapping div that changes display mode of it's switcher (e.g. dropdown_switcher_link becomes display: block although it's based on link block, that is display: inline). I propose to get rid of this root node, make dropdown as mix to switcher and popup and link it by id js-parameter, so we can use dropdown as inline element without need of writing additional styles for it. To be more exact, dropdown will inherit display mode of it's switcher after this change.