Add simple desktop app

desktop/lib/platform.dart

@@ -0,0 +1,66 @@
+import 'dart:io';
+
+import 'package:file_chooser/file_chooser.dart';
+import 'package:musicus_common/musicus_common.dart';
+import 'package:path/path.dart' as p;
+
+class MusicusDesktopPlatform extends MusicusPlatform {
+  @override
+  Future<String> chooseBasePath() async {
+    final result = await showOpenPanel(
+      canSelectDirectories: true,
+    );
+
+    if (result != null && !result.canceled) {
+      return result.paths.first;
+    } else {
+      return null;
+    }
+  }
+
+  @override
+  Future<List<Document>> getChildren(String parentId) async {
+    final List<Document> result = [];
+
+    final parent = Directory(parentId ?? basePath);
+    await for (final fse in parent.list()) {
+      result.add(Document(
+        id: fse.path,
+        name: p.basename(fse.path),
+        parent: parentId,
+        isDirectory: fse is Directory,
+      ));
+    }
+
+    return result;
+  }
+
+  @override
+  Future<String> getIdentifier(String parentId, String fileName) async {
+    return p.absolute(parentId, fileName);
+  }
+
+  @override
+  Future<String> readDocument(String id) async {
+    try {
+      return await File(id).readAsString();
+    } on FileSystemException {
+      return null;
+    }
+  }
+
+  @override
+  Future<String> readDocumentByName(String parentId, String fileName) async {
+    try {
+      return await File(p.absolute(parentId, fileName)).readAsString();
+    } on FileSystemException {
+      return null;
+    }
+  }
+
+  @override
+  Future<void> writeDocumentByName(
+      String parentId, String fileName, String contents) async {
+    await File(p.absolute(parentId, fileName)).writeAsString(contents);
+  }
+}