Give our threads names (#4356)

* Give our threads names

It helps with debugging.

* Use Q_OBJECT macro to automatically name threads.

By default, QThread sets its name based on the Qt meta class. To get an
accurate metaclass, the class which inherits QThread must declare
Q_OBJECT in its header. Futhermore, Qt's MOC requires that a Qt type be
the primary base class when declaring Q_OBJECT, hence the order of
base classes has been rearranged for some classes.

src/core/Mixer.cpp

@@ -1090,6 +1090,7 @@ Mixer::fifoWriter::fifoWriter( Mixer* mixer, fifo * _fifo ) :
 	m_fifo( _fifo ),
 	m_writing( true )
 {
+	setObjectName("Mixer::fifoWriter");
 }