Add "%s selected" subtitle to Conversation List batch mode.

While looking how to implement the action bar in the coversation itself, I had a look at how it was done in the conversation list and noticed that the xml there was containing code to show how many items currently are selected - but this code was never used.

Also, this can be done simply by mode.setTitle and mode.setSubtitle, no need for a whole xml. So I removed the xml and implemented the code.

Now, when more than 1 item is selected, the subtitle shows "%s selected" (new string to be translated).