Fix sprite lib animation: move load into LibraryItem

The `LibraryItem` component has been split into a container, responsible
for handling interaction with the storage module, and a component,
responsible for presentation. The component now always takes an
`iconURI` and the logic to load an icon from storage has been moved from
`Library` into `LibraryItem`.

TODO: move animation logic from `SpriteLibrary` into `LibraryItem`. This
should simplify the code quite a bit and might also improve performance.