Was using the wrong variable.

Don't create an intermediate Song instance when all we want is a
DetachedSong.

Fixes potential nullptr dereference.

Fixes clang warning.

Fixes potential nullptr dereference.

Fixes clang warning.

Don't duplicate the MIME type when it gets passed to a std::string.

This pointer was allocated by libc, not by GLib.

Automate the life cycle, making the code safer.

Replaces "m_title" and "m_props".  More bloat removed.

Reduce bloat.

Reduce bloat.

There is no use in duplicating these containers.  It only means that
we have to search both, duplicating the code.

Don't fail silently.

Call titleToPathElt() only once for each object.

We need this only for path names.

Not necessary, not worth the check.

Contrary to setTerminateAndWait()'s documentation, the method does
wait for worker threads to exit via pthread_join().