[nemo-qml-plugin-contacts] Export the SeasideCache instance

The cache should be the parent of item data objects
[nemo-qml-plugin-contacts] Remove duplication of QContact object

For Person objects that represent a cacheItem, use the contact object
held by the cache item.
[nemo-qml-plugin-contacts] Do not create filter keys unless used

filterKey storage is only required when using the filtered model.