Cupertino gallery app (#2715)

Resolves #2519 

Part of the larger effort to bring the samples/repo into a stable and
maintainable condition that _also_ remains useful. See: #2409

## 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 have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

cupertino_gallery/macos/Runner/Assets.xcassets/AppIcon.appiconset/Contents.json

@@ -0,0 +1,68 @@
+{
+  "images" : [
+    {
+      "size" : "16x16",
+      "idiom" : "mac",
+      "filename" : "app_icon_16.png",
+      "scale" : "1x"
+    },
+    {
+      "size" : "16x16",
+      "idiom" : "mac",
+      "filename" : "app_icon_32.png",
+      "scale" : "2x"
+    },
+    {
+      "size" : "32x32",
+      "idiom" : "mac",
+      "filename" : "app_icon_32.png",
+      "scale" : "1x"
+    },
+    {
+      "size" : "32x32",
+      "idiom" : "mac",
+      "filename" : "app_icon_64.png",
+      "scale" : "2x"
+    },
+    {
+      "size" : "128x128",
+      "idiom" : "mac",
+      "filename" : "app_icon_128.png",
+      "scale" : "1x"
+    },
+    {
+      "size" : "128x128",
+      "idiom" : "mac",
+      "filename" : "app_icon_256.png",
+      "scale" : "2x"
+    },
+    {
+      "size" : "256x256",
+      "idiom" : "mac",
+      "filename" : "app_icon_256.png",
+      "scale" : "1x"
+    },
+    {
+      "size" : "256x256",
+      "idiom" : "mac",
+      "filename" : "app_icon_512.png",
+      "scale" : "2x"
+    },
+    {
+      "size" : "512x512",
+      "idiom" : "mac",
+      "filename" : "app_icon_512.png",
+      "scale" : "1x"
+    },
+    {
+      "size" : "512x512",
+      "idiom" : "mac",
+      "filename" : "app_icon_1024.png",
+      "scale" : "2x"
+    }
+  ],
+  "info" : {
+    "version" : 1,
+    "author" : "xcode"
+  }
+}