* CachingFileStorage: Added more callbacks to provide user with more information what's going on

* Changed TestCacheSupervisor for easier use of the many callbacks
* Adapted tests for new callbacks

* GroupBaseActivity: Added sync menu command
* Preferences: Added option to enable/disable offline caching
* App: don't lock database when user wants to reload. This is done in PasswordActivity and should be done there after the password was filled into the pw field
* CheckDatabaseForChanges.cs: used when syncing a non-cached database

src/Kp2aBusinessLogic/database/CheckDatabaseForChanges.cs

@@ -0,0 +1,74 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Security.Cryptography;
+using System.Text;
+using Android.App;
+using Android.Content;
+using KeePassLib.Cryptography;
+using KeePassLib.Serialization;
+using KeePassLib.Utility;
+using keepass2android.Io;
+
+namespace keepass2android
+{
+	public class CheckDatabaseForChanges: RunnableOnFinish
+	{
+		private readonly Context _context;
+		private readonly IKp2aApp _app;
+
+
+		public CheckDatabaseForChanges(Context context, IKp2aApp app, OnFinish finish)
+			: base(finish)
+		{
+			_context = context;
+			_app = app;
+		}
+
+		public override void Run()
+		{
+			try
+			{
+				IOConnectionInfo ioc = _app.GetDb().Ioc;
+				IFileStorage fileStorage = _app.GetFileStorage(ioc);
+				if (fileStorage is CachingFileStorage)
+				{
+					throw new Exception("Cannot sync a cached database!");
+				}
+				StatusLogger.UpdateMessage(UiStringKey.CheckingDatabaseForChanges);
+				
+				//download file from remote location and calculate hash:
+				StatusLogger.UpdateSubMessage(_app.GetResourceString(UiStringKey.DownloadingRemoteFile));
+				
+
+				MemoryStream remoteData = new MemoryStream();
+				using (
+					HashingStreamEx hashingRemoteStream = new HashingStreamEx(fileStorage.OpenFileForRead(ioc), false,
+																				new SHA256Managed()))
+				{
+					hashingRemoteStream.CopyTo(remoteData);
+					hashingRemoteStream.Close();
+					
+					if (!MemUtil.ArraysEqual(_app.GetDb().KpDatabase.HashOfFileOnDisk, hashingRemoteStream.Hash))
+					{
+						_app.TriggerReload(_context);
+						Finish(true);
+					}
+					else
+					{
+						Finish(true, _app.GetResourceString(UiStringKey.RemoteDatabaseUnchanged));
+					}
+				}
+			
+				
+				
+			}
+			catch (Exception e)
+			{
+				Finish(false, e.Message);
+			}
+
+		}
+
+	}
+}