Store: architectural problems if item's id property is not "id"

[wkeese]

This problem showed up when multiple items in a List were getting the d-selected class. This was caused by deliteful/List#_createItemRenderer() calling delite/Selection#isSelected(renderItem), which
called delite/Store#getIdentity(renderItem), which returned undefined.

Why did getIdentity() return undefined? Because it was expecting a store item rather than a render item, and the render item and store item were storing the identity in a different property. The render item was {id: "foo"} whereas the corresponding store item was {value: "foo"}. (Note that the store had idProperty: "value".)

So the problem starts with delite/Store#getIdentity(item). The documentation doesn't specify whether item is a store item or a render item. But it's worse than that: sometimes getIdentity() is called on store items, and other times it's called on render items.

1. delite/StoreMap#itemToRenderItem() depends on getIdentity() working on store items.
2. deliteful/List#_createItemRenderer() calls delite/Selection#isSelected(renderItem).
   delite/Selection#isSelected() calls delite/Store#getIdentity(renderItem).

Note that delite/Selection doesn’t seem to have an idea of the difference between store items and render items. And Combobox sets Selection#selectedItem to the renderItem. So Selection#getIdentity(item) should probably also take a renderItem. But Selection#getIdentity() and Store#getIdentity() are the same method because List extends both Store and Selection.

Finally, note that StoreMap#itemToRenderItem() hardcodes the render item's id property to be called id.