Use a thread pool for the threaded music loader.
Large music libraries can cause dozens/hundreds or even more of threads
to be created at once. But all we really need is to do the loading off
of the GUI thread, and the I/O will be the bottleneck no matter how many
threads we use. So use Qt Concurrent to manage a threadpool instead,
which also simplifies the code somewhat.
I also fixed the broken global status updating when using threaded
loader while refactoring to support this.