Add the concept of Folder Settings

This contains the default editor / view / sortingMode / etc. For now the
folder settings are shared among all Folders. But the idea is that we
should be able to set different settings per folder (real or virtual)

lib/core/notes_folder_fs.dart

@@ -341,4 +341,14 @@ class NotesFolderFS with NotesFolderNotifier implements NotesFolder {
 
     return null;
   }
+
+  @override
+  NotesFolderConfig get config {
+    return NotesFolderConfig.fromSettings();
+  }
+
+  @override
+  set config(NotesFolderConfig conf) {
+    conf.save();
+  }
 }