PrefsPanel::Factory is a specialization of std::function...

... We don't need to define a class for it

src/prefs/QualityPrefs.cpp

@@ -277,11 +277,12 @@ bool QualityPrefs::Commit()
    return true;
 }
 
-PrefsPanel *QualityPrefsFactory::operator () (wxWindow *parent, wxWindowID winid)
+PrefsPanel::Factory
+QualityPrefsFactory = [](wxWindow *parent, wxWindowID winid)
 {
    wxASSERT(parent); // to justify safenew
    return safenew QualityPrefs(parent, winid);
-}
+};
 
 sampleFormat QualityPrefs::SampleFormatChoice()
 {