Added unit tests (all passing unfortunately)

UI improvements
Write mode still doesn't work

ModernKeePassLib.Test/Utility/GfxUtilTests.cs

@@ -0,0 +1,32 @@
+using NUnit.Framework;
+using System;
+
+#if KeePassLib
+using KeePassLib.Utility;
+#else
+using Splat;
+using ModernKeePassLibPCL.Utility;
+#endif
+
+namespace ModernKeePassLib.Test.Shared.Utility
+{
+    [TestFixture ()]
+    public class GfxUtilTests
+    {
+        // 16x16 all white PNG file, base64 encoded
+        const string testImageData =
+            "iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAIAAACQkWg2AAAACXBIWXMAAAsTAAA" +
+            "LEwEAmpwYAAAAB3RJTUUH3wMOFgIgmTCUMQAAABl0RVh0Q29tbWVudABDcmVhdG" +
+            "VkIHdpdGggR0lNUFeBDhcAAAAaSURBVCjPY/z//z8DKYCJgUQwqmFUw9DRAABVb" +
+            "QMdny4VogAAAABJRU5ErkJggg==";
+
+        [Test ()]
+        public void TestLoadImage ()
+        {
+            var testData = Convert.FromBase64String (testImageData);
+            var image = GfxUtil.LoadImage (testData);
+            Assert.That (image.Width, Is.EqualTo (16));
+            Assert.That (image.Height, Is.EqualTo (16));
+        }
+    }
+}