Fix convo viewer sometimes scrolling past the first unread message.

ConversationListBox was scrolling to the first expanded row in insertion
order, not necessarily the first in display order. This fixes that.

* src/client/conversation-viewer/conversation-list-box.vala
  (ConversationListBox::load_conversation): Compare expanded rows by sort
  order when inserting them, rather than by just taking the first one
  encountered. Delay loading it first until all have been inserted, so we
  know we are actually loading the first one first.