Desktop photo search (#174)

experimental/desktop_photo_search/README.md

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+# Photo Search app
+
+This macOS application enables you to search [Unsplash](https://unsplash.com/)
+for photographs that interest you. To use it, you need to add an
+**Access Key** from [Unsplash API](https://unsplash.com/developers) to
+[unsplash_access_key.dart](lib/unsplash_access_key.dart).
+
+## A quick tour of the code
+
+This Flutter project builds a desktop application. It utilises the following
+desktop specific plugins:
+
+  - [file_choser] to enable the application user to select where to save a photo
+    from the Unsplash API.
+  - [menubar] for exposing Image Search functionality through the menu bar.
+  - [url_launcher_fde] and [url_launcher] plugin, which are used to open external links.
+
+The Unsplash API client entry point is in the [Unsplash] class, and is built
+atop [http], [built_value] and [built_collection] for JSON Rest API access.
+
+The [DataTreeNode] and widget family, along with the [Split] widget capture
+desktop file explorer master/detail view idioms.
+
+## macOS Network and File entitlements
+
+To access the network, macOS requires applications enable the
+[com.apple.security.network.client entitlement][macOS-client]. For this
+sample, this entitlement is required to access the Unsplash API.
+
+Likewise, to save a Photo to the local file system using the `file_choser` plugin requires the
+[com.apple.security.files.user-selected.read-write entitlement][macOS-read-write].
+
+Please see [macOS Signing and Security][macOS-security] for more detail.
+
+## Flutter Desktop is not in Flutter Stable Release Channel
+
+This sample is an initial preview, intended to enable developers to preview what is
+under development. As such, it is currently only available for use on the `master` channel
+of Flutter. Please see [Flutter build release channels][flutter_channels] for more detail,
+and how to switch between Flutter release channels.
+
+[DataTreeNode]: lib/src/widgets/data_tree.dart
+[Split]: lib/src/widgets/split.dart
+[Unsplash]: lib/src/unsplash/unsplash.dart
+
+[built_collection]: https://pub.dev/packages/built_collection
+[built_value]: https://pub.dev/packages/built_value
+[file_choser]: https://github.com/google/flutter-desktop-embedding/tree/master/plugins/file_chooser
+[flutter_channels]: https://github.com/flutter/flutter/wiki/Flutter-build-release-channels
+[http]: https://pub.dev/packages/http
+[macOS-client]: https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_security_network_client
+[macOS-read-write]: https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_security_files_user-selected_read-write
+[macOS-security]: https://github.com/google/flutter-desktop-embedding/blob/master/macOS-Security.md
+[menubar]: https://github.com/google/flutter-desktop-embedding/tree/master/plugins/menubar
+[url_launcher]: https://pub.dev/packages/url_launcher
+[url_launcher_fde]: https://github.com/google/flutter-desktop-embedding/tree/master/plugins/flutter_plugins/url_launcher_fde