Decode folder hierarchy

app/src/main/java/eu/faircode/email/DB.java

@@ -50,7 +50,7 @@ import io.requery.android.database.sqlite.RequerySQLiteOpenHelperFactory;
 // https://developer.android.com/topic/libraries/architecture/room.html
 
 @Database(
-        version = 59,
+        version = 60,
         entities = {
                 EntityIdentity.class,
                 EntityAccount.class,
@@ -655,6 +655,13 @@ public abstract class DB extends RoomDatabase {
                         db.execSQL("CREATE  INDEX `index_contact_avatar` ON `contact` (`avatar`)");
                     }
                 })
+                .addMigrations(new Migration(59, 60) {
+                    @Override
+                    public void migrate(SupportSQLiteDatabase db) {
+                        Log.i("DB migration from version " + startVersion + " to " + endVersion);
+                        db.execSQL("ALTER TABLE `folder` ADD COLUMN `parent` INTEGER");
+                    }
+                })
                 .build();
     }