Add WebView.CustomCss for CSS style injection

TestApps/Samples/Samples/WebViewSample.cs

@@ -131,6 +131,8 @@ namespace Samples
 				"Lorem ipsum dolor sit amet.</p>" +
 				"</body></html>";
 			wb.LoadHtml (html, "sample.html");
+			wb.CustomCss =
+				"body {\n    color: red;\n    background-color: black;\n}";
 		}
 
 		public void UpdateProgress (object sender, ElapsedEventArgs args)

Xwt.Gtk.Mac/WebViewBackend.cs

@@ -84,6 +84,8 @@ namespace Xwt.Gtk.Mac
 
 		public bool ContextMenuEnabled { get; set; }
 
+		public string CustomCss { get; set; }
+
 		public void GoBack ()
 		{
 			view.GoBack ();

Xwt.Gtk.Windows/WebViewBackend.cs

@@ -118,6 +118,8 @@ namespace Xwt.Gtk.Windows
 
 		public bool ContextMenuEnabled { get; set; }
 
+		public string CustomCss { get; set; }
+
 		public void GoBack ()
 		{
 			view.GoBack ();

Xwt.Gtk/Xwt.GtkBackend/WebViewBackend.cs

@@ -82,6 +82,8 @@ namespace Xwt.GtkBackend
 
 		public bool ContextMenuEnabled { get; set; }
 
+		public string CustomCss { get; set; }
+
 		public void GoBack ()
 		{
 			view.GoBack ();

Xwt.WPF/Xwt.WPFBackend/WebViewBackend.cs

@@ -82,6 +82,8 @@ namespace Xwt.WPFBackend
 
 		public bool ContextMenuEnabled { get; set; }
 
+		public string CustomCss { get; set; }
+
 		public void GoBack ()
 		{
 			view.GoBack ();

Xwt.XamMac/Xwt.Mac/WebViewBackend.cs

@@ -97,6 +97,8 @@ namespace Xwt.Mac
 
 		public bool ContextMenuEnabled { get; set; }
 
+		public string CustomCss { get; set; }
+
 		public void GoBack ()
 		{
 			Widget.GoBack ();

Xwt/Xwt.Backends/IWebViewBackend.cs

@@ -41,6 +41,7 @@ namespace Xwt.Backends
 		void StopLoading ();
 		void LoadHtml (string content, string base_uri);
 		bool ContextMenuEnabled { get; set; }
+		string CustomCss { get; set; }
 	}
 
 	public interface IWebViewEventSink : IWidgetEventSink

Xwt/Xwt/WebView.cs

@@ -132,6 +132,12 @@ namespace Xwt
 			set { Backend.ContextMenuEnabled = value; }
 		}
 
+		[DefaultValue (null)]
+		public string CustomCss {
+			get { return Backend.CustomCss; }
+			set { Backend.CustomCss = value; }
+		}
+
 		public void GoBack ()
 		{
 			Backend.GoBack ();