_trimTextContent (el) {
    var nodes = el.getChildNodes()
    var firstNode = nodes[0]
    var lastNode = last(nodes)
    var text, trimmed
    // trim the first and last text
    if (firstNode && firstNode.isTextNode()) {
      text = firstNode.textContent
      trimmed = this._trimLeft(text)
      firstNode.textContent = trimmed
    }
    if (lastNode && lastNode.isTextNode()) {
      text = lastNode.textContent
      trimmed = this._trimRight(text)
      lastNode.textContent = trimmed
    }
    return el
  }
Removes any leading and trailing whitespaces from the content
within the given element.
Attention: this is not yet implemented fully. Atm, trimming is only done
on the first and last text node (if they exist).