This allows a reference to be returned (without wrapping all subitems in
a unique_ptr or a reference_wrapper, or a manually heap allocated Item*)

Incidentally, this fixes a missing file_id update when associating a
file with a playlist

This involves an extra & useless dereference, and we end up copying the
shared_ptr anyway.
Might as well copy & move it when done.

This is solely replacing direct members access with a pair of
getters/setters

Fix #35

Instead of potentially using a thumbnail from an album the artist is
only featuring on

It is now up to the user to request thumbnails generation.
It allows lazy loading, and prevents us from potentially generating
thousands of thumbnails for nothing when indexing an entire network
drive.

We now fallback to starting a playback early, no need for the old
complicated and full of special case code