merge leading/trailing standalone elements into wrappers

[jenseng]

Another thing for nicer translation strings... Consider:

<button onClick={this.close}>
  <Icon className="i-close" />
  <span translate="yes">Close</span>
</button>

The span is superfluous, but if we put translate="yes" on the <button /> then translators will see a placeholder for the <Icon />, which is not helpful.

If a wrapper has leading or trailing elements with no text content, we should absorb them into the wrapper. Then you can safely do:

<button onClick={this.close} translate="yes">
  <Icon className="i-close" />
  Close
</button>

an the extracted string will just be:

"Close"

i18nliner-handlebars and canvas_react_i18n both do this, we should too.