Bug 1003843 - Follow up to Bug 951676 - [Messages][Refresh] Add specific image in case of more than one recipient in the list threads. r=schung

I'm really upset that we can't have only one element for this (now we have aside->span[data-type=img]) because of BB list styling. Currently I see two options:
1. The current one, assign ```.threadlist-item-picture``` to ```span[data-type=img]```, but use ```parentNode``` to hide ```aside``` when we don't need it. It simplifies CSS and looks reasonable, but we have ugly ```parentNode``` dependency;
2. Add ```.threadlist-item-picture``` for ```aside``` itself. In this case we'll have css rules like ```.threadlist-item-picture > span[data-type=img]```, but will get rid of ```parentNode``` and ```.hide```.

What do you think? Maybe we have other better options...