Changed VMs references to database singleton

Added some unit tests (WIP)

ModernKeePass/ViewModels/SaveVm.cs

@@ -1,20 +1,27 @@
 using Windows.Storage;
 using Windows.UI.Xaml;
+using ModernKeePass.Interfaces;
 
 namespace ModernKeePass.ViewModels
 {
     public class SaveVm
     {
+        private readonly IDatabase _database;
+        public SaveVm() : this((Application.Current as App)?.Database) { }
+
+        public SaveVm(IDatabase database)
+        {
+            _database = database;
+        }
+
         public void Save(bool close = true)
         {
-            var app = (App)Application.Current;
-            if (close && app.Database.Save()) app.Database.Close();
+            if (close && _database.Save()) _database.Close();
         }
 
         internal void Save(StorageFile file)
         {
-            var app = (App)Application.Current;
-            app.Database.Save(file);
+            _database.Save(file);
         }
     }
 }