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

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

src/prefs/GUIPrefs.cpp

@@ -268,8 +268,9 @@ bool GUIPrefs::Commit()
    return true;
 }
 
-PrefsPanel *GUIPrefsFactory::operator () (wxWindow *parent, wxWindowID winid)
+PrefsPanel::Factory
+GUIPrefsFactory = [](wxWindow *parent, wxWindowID winid)
 {
    wxASSERT(parent); // to justify safenew
    return safenew GUIPrefs(parent, winid);
-}
+};