updated preview assets

Author: gitbrent

MacImageManager/MacImageManager/Assets.xcassets/Preview/Preview1.imageset/Preview1.png

@@ -1,7 +1,7 @@
 {
   "images" : [
     {
-      "filename" : "Preview1.png",
+      "filename" : "preview1.png",
       "idiom" : "universal",
       "scale" : "1x"
     },

MacImageManager/MacImageManager/Assets.xcassets/Preview/Preview2.imageset/Preview2.png

@@ -1,7 +1,7 @@
 {
   "images" : [
     {
-      "filename" : "Preview2.png",
+      "filename" : "preview2.png",
       "idiom" : "universal",
       "scale" : "1x"
     },

MacImageManager/MacImageManager/Assets.xcassets/Preview/Preview1.imageset/Contents.json renamed to MacImageManager/MacImageManager/Assets.xcassets/Preview/preview1.imageset/Contents.json

@@ -0,0 +1,21 @@
+{
+  "images" : [
+    {
+      "filename" : "preview3.png",
+      "idiom" : "universal",
+      "scale" : "1x"
+    },
+    {
+      "idiom" : "universal",
+      "scale" : "2x"
+    },
+    {
+      "idiom" : "universal",
+      "scale" : "3x"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}