introduce auto save option (#30)

* introduce auto save every minute
* implement loading from AutoSave file
* fix threading for loading auto.save on application start; cleanup
* use Xunit.StaFact for testing using UI-dispatcher-like thread

src/SimpleAccounting/Abstractions/FileSystem.cs

@@ -4,8 +4,10 @@
 
 namespace lg2de.SimpleAccounting.Abstractions
 {
+    using System;
     using System.Diagnostics.CodeAnalysis;
     using System.IO;
+    using System.Text;
 
     /// <summary>
     ///     Redirects <see cref="IFileSystem"/> into real implementations from .NET framework.
@@ -21,5 +23,30 @@ public bool FileExists(string filePath)
         {
             return File.Exists(filePath);
         }
+
+        public void FileMove(string sourceFileName, string destFileName)
+        {
+            File.Move(sourceFileName, destFileName);
+        }
+
+        public void FileDelete(string path)
+        {
+            File.Delete(path);
+        }
+
+        public DateTime GetLastWriteTime(string path)
+        {
+            return File.GetLastWriteTime(path);
+        }
+
+        public void WriteAllTextIntoFile(string path, string content)
+        {
+            File.WriteAllText(path, content, Encoding.UTF8);
+        }
+
+        public string ReadAllTextFromFile(string path)
+        {
+            return File.ReadAllText(path);
+        }
     }
 }

src/SimpleAccounting/Abstractions/IFileSystem.cs

@@ -4,6 +4,8 @@
 
 namespace lg2de.SimpleAccounting.Abstractions
 {
+    using System;
+
     /// <summary>
     ///     Defines abstraction for file system access not available in .NET framework.
     /// </summary>
@@ -15,5 +17,43 @@ internal interface IFileSystem
         /// <param name="filePath">The path to the file to be checked.</param>
         /// <returns>Returns <c>true</c> if the file exists.</returns>
         bool FileExists(string filePath);
+
+        /// <summary>
+        ///     Moves a specified file to a new location, providing the option to specify a new file name.
+        /// </summary>
+        /// <param name="sourceFileName">The name of the file to move. Can include a relative or absolute path.</param>
+        /// <param name="destFileName">The new path and name for the file.</param>
+        void FileMove(string sourceFileName, string destFileName);
+
+        /// <summary>
+        ///     Deletes the specified file.
+        /// </summary>
+        /// <param name="path">The name of the file to be deleted. Wildcard characters are not supported.</param>
+        void FileDelete(string path);
+
+        /// <summary>
+        ///     Returns the date and time the specified file or directory was last written to.
+        /// </summary>
+        /// <param name="path">The file or directory for which to obtain write date and time information.</param>
+        /// <returns>
+        ///     A DateTime structure set to the date and time that the specified file or directory was last written to.
+        ///     This value is expressed in local time.
+        /// </returns>
+        DateTime GetLastWriteTime(string path);
+
+        /// <summary>
+        ///     Creates a new file, writes the specified string to the file, and then closes the file.
+        /// If the target file already exists, it is overwritten.
+        /// </summary>
+        /// <param name="path">The file to write to.</param>
+        /// <param name="content">The string to write to the file.</param>
+        void WriteAllTextIntoFile(string path, string content);
+
+        /// <summary>
+        ///     Opens a text file, reads all the text in the file into a string, and then closes the file.
+        /// </summary>
+        /// <param name="path">The file to open for reading.</param>
+        /// <returns>A string containing all the text in the file.</returns>
+        string ReadAllTextFromFile(string path);
     }
 }

src/SimpleAccounting/App.xaml.cs

@@ -4,23 +4,19 @@
 
 namespace lg2de.SimpleAccounting
 {
-    using System.Collections.Specialized;
     using System.Diagnostics.CodeAnalysis;
     using System.Globalization;
     using System.Windows;
     using System.Windows.Markup;
     using lg2de.SimpleAccounting.Properties;
 
     [ExcludeFromCodeCoverage]
-    public partial class App : Application
+    public partial class App
     {
         public App()
         {
-            Settings.Default.Upgrade();
-            if (Settings.Default.RecentProjects == null)
-            {
-                Settings.Default.RecentProjects = new StringCollection();
-            }
+            var settings = Settings.Default;
+            settings.Upgrade();
         }
 
         private void ApplicationStartup(object sender, StartupEventArgs e)