Look into caching with Keyer and Fetcher

[KevinSchildhorn]

In fetcher docs it mentions that:

If you add a Fetcher that uses a custom data type, you need to also need to provide a custom Keyer to ensure results from requests that use it are memory cacheable. For example, Fetcher.Factory will need to add a Keyer.

Should look into this to replace the strange caching implementation currently in FotoPresenter.