Make ProgressBar compile.

And add to test app.

Perspex.Themes.Default/ProgressBarStyle.cs

@@ -36,7 +36,7 @@
                 [~Border.BorderBrushProperty] = control[~ProgressBar.BorderBrushProperty],
                 [~Border.BorderThicknessProperty] = control[~ProgressBar.BorderThicknessProperty],
 
-                Content = new Grid
+                Child = new Grid
                 {
                     MinHeight = 14,
                     MinWidth = 200,
@@ -56,8 +56,7 @@
                             BorderThickness = 1,
                             HorizontalAlignment = HorizontalAlignment.Left,
                             [~Border.BackgroundProperty] = control[~ProgressBar.ForegroundProperty],
-                           
+                            Child = new Grid
-                            Content = new Grid
                             {
                                 Name = "Animation",
                                 ClipToBounds = true,

TestApplication/Program.cs

@@ -448,6 +448,10 @@ namespace TestApplication
                                 [!Image.HeightProperty] = size[!ScrollBar.ValueProperty],
                             },
                         },
+                        new ProgressBar
+                        {
+                            Value = 50
+                        }
                     }
                 },
             };