Add LoadApplication method

This allows a WASM Ooui app to act like a native Forms app.

Fixes #106

Ooui.Forms/Forms.cs

@@ -150,5 +150,14 @@ namespace Xamarin.Forms
             public VisualElement View { get; set; }
             public Ooui.Element NativeView { get; set; }
         }
+
+        public static void LoadApplication (Application application)
+        {
+            Application.Current = application;
+            var mainPage = application.MainPage;
+            if (mainPage != null) {
+                UI.Publish ("/", application.MainPage.GetOouiElement ());
+            }
+        }
     }
 }