Watcher on props doesn't react on deep changes

[gaperton]

• Props watchers must use the same mechanics as pureRender to detect props changes.
• state and store updates must not trigger UI updates when made from the props watcher.
• Watchers must rely on new React lifecycle hooks.

static props = {
    items : Collection.has.watcher( 'onItemsChange' )
}

The watcher is called if the new instance of items being passed, but not when something was changed inside.