Fix Font properties owner.

And make default values come from theme.

Perspex.Controls/Presenters/TextPresenter.cs

@@ -347,13 +347,16 @@ namespace Perspex.Controls
 
         internal void CaretIndexChanged(int caretIndex)
         {
-            this.caretBlink = true;
-            this.caretTimer.Stop();
-            this.caretTimer.Start();
-            this.InvalidateVisual();
+            if (this.GetVisualParent() != null)
+            {
+                this.caretBlink = true;
+                this.caretTimer.Stop();
+                this.caretTimer.Start();
+                this.InvalidateVisual();
 
-            var rect = this.FormattedText.HitTestTextPosition(caretIndex);
-            this.BringIntoView(rect);
+                var rect = this.FormattedText.HitTestTextPosition(caretIndex);
+                this.BringIntoView(rect);
+            }
         }
 
         private void MoveHorizontal(int count, ModifierKeys modifiers)

Perspex.Controls/TextBlock.cs

@@ -17,16 +17,13 @@ namespace Perspex.Controls
             Border.BackgroundProperty.AddOwner<TextBlock>();
 
         public static readonly PerspexProperty<string> FontFamilyProperty =
-            PerspexProperty.Register<Control, string>("FontFamily", "Segoe UI", inherits: true);
+            PerspexProperty.Register<TextBlock, string>("FontFamily", inherits: true);
 
         public static readonly PerspexProperty<double> FontSizeProperty =
-            PerspexProperty.Register<Control, double>(
-                "FontSize",
-                defaultValue: 12.0,
-                inherits: true);
+            PerspexProperty.Register<TextBlock, double>("FontSize", inherits: true);
 
         public static readonly PerspexProperty<FontStyle> FontStyleProperty =
-            PerspexProperty.Register<Control, FontStyle>("FontStyle", inherits: true);
+            PerspexProperty.Register<TextBlock, FontStyle>("FontStyle", inherits: true);
 
         public static readonly PerspexProperty<string> TextProperty =
             PerspexProperty.Register<TextBlock, string>("Text");

Perspex.Themes.Default/WindowStyle.cs

@@ -23,6 +23,8 @@ namespace Perspex.Themes.Default
                     Setters = new[]
                     {
                         new Setter(Window.TemplateProperty, ControlTemplate.Create<Window>(this.Template)),
+                        new Setter(Window.FontFamilyProperty, "Segoe UI"),
+                        new Setter(Window.FontSizeProperty, 12.0),
                     },
                 },
             });