Next Gen UI demo (#1778)

First pass at a Next Generation UI demo app. The UI needs work, feedback
gratefully accepted.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

next_gen_ui_demo/linux/flutter/generated_plugin_registrant.cc

@@ -0,0 +1,15 @@
+//
+//  Generated file. Do not edit.
+//
+
+// clang-format off
+
+#include "generated_plugin_registrant.h"
+
+#include <window_size/window_size_plugin.h>
+
+void fl_register_plugins(FlPluginRegistry* registry) {
+  g_autoptr(FlPluginRegistrar) window_size_registrar =
+      fl_plugin_registry_get_registrar_for_plugin(registry, "WindowSizePlugin");
+  window_size_plugin_register_with_registrar(window_size_registrar);
+}