popup: split own functionality into target modifiers (close #968)

`setContent` is valuable because does 2 things at the same time: updates the DOM and redraws the popup. Since we need to redraw the popup only in `_target_*` modifications, this method valuable only in this modifications. in `popup.js` it only  updates the DOM and loses value, because we can do that just through `BEMDOM.update(popup.domElem, content)`.