_startDrag(e) {
      let clientX = e.targetTouches[0].clientX;
      this.isDragged = true;
      this._startingXpos = clientX;
      this._xPos = this._startingXpos;
      this._time = Date.now();
      this._width = this.el.getBoundingClientRect().width;
      this._overlay.style.display = 'block';
      this._initialScrollTop = this.isOpen ? this.el.scrollTop : M.getDocumentScrollTop();
      this._verticallyScrolling = false;
      anim.remove(this.el);
      anim.remove(this._overlay);
    }
Set variables needed at the beggining of drag
and stop any current transition.
@param {Event} e