We now only process one file at a time, as we do for snapshots.
The correct way would be to spawn more threads from the Parser class,
shall we want to process multiple items at once

Since the discovery will run in a thread, we can't assume results are
instantly available

failed test > sigsegv

This wrapper is responsible for stacking operations and running them in
a thread, rather than spawning a new std::thread each time

This removes the old std::string artist property on AlbumTrack

A file is now always "ready" in the sense that is always has a type

The file is initially determined as a best guess
This also remove images from supported extensions

This reverts commit 5ac40117.
There was no lock involved, I blame E_NOCOFEE :|

It's now true when all metadata services have been run. The type is
still exposed and can be used to figure out if the basic metadata are
available