Support for multi-select in the conversation list.

Unfortunately I'm pretty sure this is the only way to do multi-select on both <> Honeycomb. It'd be nice if we could use the new stuff they were experimenting with in #1601, but I don't think it'll be backwards compatible.
Currently it is ask for deleting *this* message. Would be better to have sth. like:
````JAVA
    if (messageRecords.size() > 1)
      builder.setMessage(R.string.ConversationFragment_are_you_sure_you_want_to_permanently_delete_these_messages);
    else
      builder.setMessage(R.string.ConversationFragment_are_you_sure_you_want_to_permanently_delete_this_message);
````