Rather than keeping a single conversation list model instance around -
attaching and adding conversations then clearing and detaching
conversation viewers, simply remove the old model instance and add a new
one.

This also allows the ConversationListStore::is_clearing property to be
removed, fixing a related critical warning, and simply unhooking the
selection-changed signal handler instead when changing the model.

* src/client/components/main-window.vala (MainWindow): Remove props that
  are now dynamically managed. When the conversation monitor changes,
  replace the ConversationListView's model, update progress listeners.

* src/client/conversation-list/conversation-list-view.vala
  (ConversationListView): Remove list store property since the parent
  class already maintains it. Override ::get_model() and ::set_model() to
  use ConversationListStore instances, manage signals on model instances
  when they are set/unset.

* src/client/conversation-list/conversation-list-store.vala
  (ConversationListStore): Replae destructor with an explict destroy
  method that ensures the object is cleaned up enough to actualy be
  finalised. Remove use of is_clearing in ::on_selection_changed() to fix
  critical warning.