Destroy item's of last pane even if the pane itself is not destroyed

We always want to preserve at least one pane, so we don't call destroy
on the pane if it is the last pane. However, we still want to destroy
its items. This was previously relying on the view to destroy the
underlying item, which isn't as reliable as doing it in the model.

src/pane.coffee

@@ -212,7 +212,10 @@ class Pane extends Model
     @destroyItem(item) for item in @getItems() when item isnt @activeItem
 
   destroy: ->
-    super unless @container?.isAlive() and @container?.getPanes().length is 1
+    if @container?.isAlive() and @container.getPanes().length is 1
+      @destroyItems()
+    else
+      super
 
   # Called by model superclass.
   destroyed: ->