Implement Write() in Config and rename DatabasePath to DatabaseFile

Elwig/App.xaml.cs

@@ -26,7 +26,7 @@ namespace Elwig {
         protected override void OnStartup(StartupEventArgs evt) {
             using (var ctx = new AppDbContext()) {
                 if (!ctx.Database.CanConnect()) {
-                    MessageBox.Show($"Invalid Database:\n\n{Config.DatabasePath}", "Invalid Database", MessageBoxButton.OK, MessageBoxImage.Error);
+                    MessageBox.Show($"Invalid Database:\n\n{Config.DatabaseFile}", "Invalid Database", MessageBoxButton.OK, MessageBoxImage.Error);
                     Shutdown();
                 }
             }