Re-enabled write mode in KeePassLib

Set database as part of App (so, Singleton)
Changed main page view model
WIP and testing saving mechanism

ModernKeePass/Common/DatabaseHelper.cs

@@ -0,0 +1,59 @@
+using System;
+using Windows.Storage;
+using System.Threading.Tasks;
+
+using ModernKeePass.ViewModels;
+using ModernKeePassLib;
+using ModernKeePassLib.Interfaces;
+using ModernKeePassLib.Keys;
+using ModernKeePassLib.Serialization;
+
+namespace ModernKeePass.Common
+{
+    public class DatabaseHelper
+    {
+        private PwDatabase _pwDatabase = new PwDatabase();
+        private StorageFile databaseFile;
+
+        public GroupVm RootGroup { get; set; }
+        public bool IsOpen { get; set; }
+        public string Name { get; set; }
+
+        public DatabaseHelper(StorageFile databaseFile)
+        {
+            this.databaseFile = databaseFile;
+        }
+        public async Task<string> Open(string password)
+        {
+            var key = new CompositeKey();
+            try
+            {
+                key.AddUserKey(new KcpPassword(password));
+                await _pwDatabase.Open(IOConnectionInfo.FromFile(databaseFile), key, new NullStatusLogger());
+                IsOpen = _pwDatabase.IsOpen;
+                Name = databaseFile.DisplayName;
+                RootGroup = new GroupVm(_pwDatabase.RootGroup);
+            }
+            catch (ArgumentNullException)
+            {
+                return "Password cannot be empty";
+            }
+            catch (InvalidCompositeKeyException)
+            {
+                return "Wrong password";
+            }
+            /*finally
+            {
+                // TODO: move this when implementing write mode
+                _pwDatabase.Close();
+            }*/
+            return string.Empty;
+        }
+
+        public void Save()
+        {
+            _pwDatabase.Save(new NullStatusLogger());
+            _pwDatabase.Close();
+        }
+    }
+}