InstrumentTrackWindow: clear focus on closeEvent

When closing an InstrumentTrackWindow make sure it doesn't still have
keyboard focus (looks like a bug in Qt) and set focus on
trackLabelButton instead (closes #2780590).

src/tracks/instrument_track.cpp

@@ -1430,6 +1430,7 @@ void instrumentTrackWindow::closeEvent( QCloseEvent * _ce )
 	{
 		hide();
 	}
+	m_itv->m_tlb->setFocus();
 	m_itv->m_tlb->setChecked( false );
 }