Apply code review feedback to Quick Info PR

This commit has a few significant changes beyond correcting minor nit picks:

1. It unifies the GlyphExtensions classes in the Workspaces and non-WPF EditorFeatures layers.
2. It significantly cleans up the super-ugly mocking code in QuickInfoControlTests.vb
3. Tweaks the QuickInfo API to remove QuickInfoItem.Empty and QuickInfoItem.IsEmpty properties.

src/EditorFeatures/CSharpTest/QuickInfo/SemanticQuickInfoSourceTests.cs

@@ -55,14 +55,15 @@ private async Task TestWithOptionsAsync(TestWorkspace workspace, params Action<Q
         private async Task TestWithOptionsAsync(Document document, QuickInfoService service, int position, Action<QuickInfoItem>[] expectedResults)
         {
             var info = await service.GetQuickInfoAsync(document, position, cancellationToken: CancellationToken.None);
-            Assert.NotNull(info);
 
             if (expectedResults.Length == 0)
             {
-                Assert.True(info.IsEmpty);
+                Assert.Null(info);
             }
             else
             {
+                Assert.NotNull(info);
+
                 foreach (var expected in expectedResults)
                 {
                     expected(info);
@@ -91,14 +92,14 @@ private async Task VerifyWithMscorlib45Async(string markup, Action<QuickInfoItem
 
                 var info = await service.GetQuickInfoAsync(document, position, cancellationToken: CancellationToken.None);
 
-                Assert.NotNull(info);
-
                 if (expectedResults.Length == 0)
                 {
-                    Assert.True(info.IsEmpty);
+                    Assert.Null(info);
                 }
                 else
                 {
+                    Assert.NotNull(info);
+
                     foreach (var expected in expectedResults)
                     {
                         expected(info);
@@ -237,14 +238,14 @@ private async Task VerifyWithReferenceWorkerAsync(string xmlString, params Actio
 
                 var info = await service.GetQuickInfoAsync(document, position, cancellationToken: CancellationToken.None);
 
-                Assert.NotNull(info);
-
                 if (expectedResults.Length == 0)
                 {
-                    Assert.True(info.IsEmpty);
+                    Assert.Null(info);
                 }
                 else
                 {
+                    Assert.NotNull(info);
+
                     foreach (var expected in expectedResults)
                     {
                         expected(info);

src/EditorFeatures/Core.Wpf/Completion/Presentation/ImageMonikers.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System.Collections.Immutable;
-using Microsoft.CodeAnalysis.Editor.Shared.Extensions;
 using Microsoft.CodeAnalysis.Editor.Wpf;
 using Microsoft.VisualStudio.Imaging.Interop;
 

src/EditorFeatures/Core.Wpf/Tags/DefaultImageMonikerService.cs

@@ -2,7 +2,6 @@
 
 using System.Collections.Immutable;
 using System.Composition;
-using Microsoft.CodeAnalysis.Editor.Shared.Extensions;
 using Microsoft.CodeAnalysis.Editor.Wpf;
 using Microsoft.VisualStudio.Imaging;
 using Microsoft.VisualStudio.Imaging.Interop;
@@ -16,7 +15,7 @@ internal class DefaultImageMonikerService : IImageMonikerService
 
         public bool TryGetImageMoniker(ImmutableArray<string> tags, out ImageMoniker imageMoniker)
         {
-            var glyph = tags.GetGlyph();
+            var glyph = tags.GetFirstGlyph();
 
             // We can't do the compositing of these glyphs at the editor layer.  So just map them
             // to the non-add versions.