add api culture

OwnChar.App.Desktop/Api/IOwnCharApi.cs

@@ -0,0 +1,14 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace OwnChar.App.Desktop.Api
+{
+    public interface IOwnCharApi
+    {
+        public static IOwnCharApi Instance { get; } = new AppApi();
+        public IMainWindowApi? MainWindow { get; internal set; }
+    }
+}