Cache behaviour inconsistent

[stevehayles]

The dynamic slots functionality behaves differently with regard to the cache depending on previous actions. If the cache contains one or less 'slotCounts' then when "_onItemsAdded" is called in the DashboardItemStorageDelegate the item is cached for that layout correctly but if the cache (ie. Map<int, Map<String, T>> _items) has already had values for multiple slotcounts then the "_onItemsAdded" function adds any new items to all of the slotCounts which ties their layout behaviour together.

It's not clear to me exactly what the intended functionality is meant to be but it's inconsistent right now and makes for confusing behaviour for a user. If I disable caching then I get the behaviour I would expect