Add draft concept for sync services

lib/sync/file.dart

@@ -0,0 +1,31 @@
+import 'package:flutter/cupertino.dart';
+
+abstract class RemoteFile {
+  /// File path.
+  @required
+  String path;
+
+  /// File content length or folder size
+  @required
+  int size;
+
+  // Date of last modification.
+  @required
+  DateTime lastModified;
+
+  /// Creation date of the file.
+  @required
+  DateTime createdDate;
+
+  /// Returns the decoded name of the file / folder without the whole path
+  @required
+  String get name {
+    // normalised path (remove trailing slash)
+    final end = path.endsWith('/') ? path.length - 1 : path.length;
+    return Uri.parse(path, 0, end).pathSegments.last;
+  }
+
+  /// Whether the file is a directory.
+  @required
+  bool isDirectory;
+}

lib/sync/sync.dart

@@ -0,0 +1,18 @@
+import 'package:flutter/widgets.dart';
+import 'package:txt/sync/file.dart';
+
+abstract class SyncService {
+  Future<void> authorize(BuildContext context);
+
+  Future<String> getAccountName();
+
+  Future<List<RemoteFile>> listFolder(String path);
+
+  Future<void> upload(String localPath, String remotePath);
+
+  Future<void> download(String remotePath, String localPath);
+}
+
+Future<void> sync(String localPath, String remotePath) async {
+  throw UnimplementedError();
+}