dfmanager cleanup (#6211)

* Turn `DFManager` into a singleton
* PIMPL `DFManager`
* Cleanup namespace usage in `dfmanager.cc`
* Constify `DFManager` interface
* Fix bad `reinterpret_cast` between `std::string` and `Glib::ustring`

Co-authored-by: Flössie <floessie.mail@gmail.com>
Co-authored-by: Thanatomanic <6567747+Thanatomanic@users.noreply.github.com>

rtgui/filebrowser.cc

@@ -875,7 +875,7 @@ void FileBrowser::menuItemActivated (Gtk::MenuItem* m)
                 }
 
                 // Reinit cache
-                rtengine::dfm.init( options.rtSettings.darkFramesPath );
+                rtengine::DFManager::getInstance().init( options.rtSettings.darkFramesPath );
             } else {
                 // Target directory creation failed, we clear the darkFramesPath setting
                 options.rtSettings.darkFramesPath.clear();