Added basic app state with single timer

Signed-off-by: Andreas Fahrecker <AndreasFahrecker@gmail.com>

lib/models/app_state.dart

@@ -0,0 +1,23 @@
+import 'package:meta/meta.dart';
+import 'package:time_progress_calculator/models/timer.dart';
+
+@immutable
+class AppState {
+  final Timer timer;
+
+  AppState({this.timer});
+
+  AppState copyWith({Timer timer}) {
+    return AppState(timer: timer ?? this.timer);
+  }
+
+  @override
+  int get hashCode => timer.hashCode;
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is AppState &&
+          runtimeType == other.runtimeType &&
+          timer == other.timer;
+}