[WPF] Implement new Font api

Testing/Tests/FontTests.cs

@@ -44,8 +44,8 @@ namespace Xwt
 		public void WithFamily ()
 		{
 			var f1 = Font.SystemFont;
-			var f2 = f1.WithFamily ("Courier");
-			Assert.AreEqual ("Courier", f2.Family);
+			var f2 = f1.WithFamily ("Arial");
+			Assert.AreEqual ("Arial", f2.Family);
 			Assert.AreEqual (f1.Size, f2.Size);
 			Assert.AreEqual (f1.Stretch, f2.Stretch);
 			Assert.AreEqual (f1.Style, f2.Style);

Xwt.WPF/Xwt.WPFBackend/ContextBackendHandler.cs

@@ -245,10 +245,6 @@ namespace Xwt.WPFBackend
 			c.SetPattern ((System.Windows.Media.Brush)p);
 		}
 
-		public override void SetFont (object backend, Font font)
-		{
-		}
-
 		public override void DrawTextLayout (object backend, TextLayout layout, double x, double y)
 		{
 			var c = (DrawingContext) backend;

Xwt.WPF/Xwt.WPFBackend/FontBackendHandler.cs

@@ -26,6 +26,7 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+using System.Linq;
 using SW = System.Windows;
 
 using Xwt.Backends;
@@ -37,6 +38,21 @@ namespace Xwt.WPFBackend
 {
 	public class WpfFontBackendHandler : FontBackendHandler
 	{
+		public override object GetSystemDefaultFont ()
+		{
+			double size = WpfFontBackendHandler.GetPointsFromDeviceUnits (SW.SystemFonts.MessageFontSize);
+
+			return new FontData (SW.SystemFonts.MessageFontFamily, size) {
+				Style = SW.SystemFonts.MessageFontStyle,
+				Weight = SW.SystemFonts.MessageFontWeight
+			};
+		}
+
+		public override System.Collections.Generic.IEnumerable<string> GetInstalledFonts ()
+		{
+			return System.Windows.Media.Fonts.SystemFontFamilies.Select (f => f.Source);
+		}
+
 		public override object Create (string fontName, double size, FontStyle style, FontWeight weight, FontStretch stretch)
 		{
 			return new FontData (new FontFamily (fontName), size) {

Xwt.WPF/Xwt.WPFBackend/TextLayoutBackendHandler.cs

@@ -38,17 +38,8 @@ namespace Xwt.WPFBackend
 	public class WpfTextLayoutBackendHandler
 		: TextLayoutBackendHandler
 	{
-		public override object Create (Context context)
+		public override object Create ()
 		{
-			var drawingContext = (DrawingContext)Toolkit.GetBackend (context);
-			return new TextLayoutBackend () {
-				Context = drawingContext
-			};
-		}
-
-		public override object Create (ICanvasBackend canvas)
-		{
-			var backend = (WidgetBackend)canvas;
 			return new TextLayoutBackend ();
 		}