Add “does.not.exist” 1x1 transparent image

This is to silence runtime warnings about missing images for the workaround used to add a11y labels/hints to toolbar buttons.

Shared/Assets.xcassets/does.not.exist.imageset/Contents.json

@@ -0,0 +1,23 @@
+{
+  "images" : [
+    {
+      "filename" : "does.not.exist.png",
+      "idiom" : "universal",
+      "scale" : "1x"
+    },
+    {
+      "filename" : "does.not.exist@2x.png",
+      "idiom" : "universal",
+      "scale" : "2x"
+    },
+    {
+      "filename" : "does.not.exist@3x.png",
+      "idiom" : "universal",
+      "scale" : "3x"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}