Call super in constructor to assign PaneModel properties

The theorist model superclass constructor will automatically assign
all declared properties, or substitute the specified default value if
no param key is present for that property.

src/pane-model.coffee

@@ -8,10 +8,11 @@ class PaneModel extends Model
   Serializable.includeInto(this)
 
   @properties
+    items: -> []
     activeItem: null
 
-  constructor: ({@items, @activeItem}) ->
-    @items ?= []
+  constructor: ->
+    super
     @activeItem ?= @items[0]
 
   serializeParams: ->