Add functionality to edit sheets with save and restore

lib/upload_queue.dart

@@ -0,0 +1,66 @@
+import 'dart:collection';
+
+import 'package:sheetless/sheet.dart';
+
+enum ChangeType {
+  sheetNameChange,
+  composerNameChange,
+  addTagChange,
+  removeTagChange,
+}
+
+class Change {
+  final ChangeType type;
+  final String sheetUuid;
+  final String value;
+
+  Change({required this.type, required this.sheetUuid, required this.value});
+
+  Map<String, dynamic> toMap() => {
+    "type": type.index,
+    "sheetUuid": sheetUuid,
+    "value": value,
+  };
+
+  factory Change.fromMap(Map<dynamic, dynamic> map) {
+    return Change(
+      type: ChangeType
+          .values[map["type"]], // TODO: this will create problems if new enums are added
+      sheetUuid: map["sheetUuid"],
+      value: map["value"],
+    );
+  }
+}
+
+class ChangeQueue {
+  // Queue with oldest change first
+  final Queue<Change> queue = Queue();
+
+  ChangeQueue() {}
+
+  void addChange(Change change) {
+    queue.addLast(change);
+  }
+
+  int length() {
+    return queue.length;
+  }
+
+  void applyToSheets(List<Sheet> sheets) {
+    for (Change change in queue) {
+      var sheet = sheets.where((sheet) => sheet.uuid == change.sheetUuid).first;
+      switch (change.type) {
+        case ChangeType.sheetNameChange:
+          sheet.name = change.value;
+        case ChangeType.composerNameChange:
+          sheet.composerName = change.value;
+        case ChangeType.addTagChange:
+          // TODO: Handle this case.
+          throw UnimplementedError();
+        case ChangeType.removeTagChange:
+          // TODO: Handle this case.
+          throw UnimplementedError();
+      }
+    }
+  }
+}