Changed LastPlayed field from string to nullable DateTime (#4861)

* Changed LastPlayed field from string to nullable DateTime

Added ApplicationData.LastPlayedString property
Added NullableDateTimeConverter for the DateTime->string conversion in Avalonia

* Added migration from string-based last_played to DateTime-based last_played_utc

* Updated comment style

* Added MarkupExtension to NullableDateTimeConverter and changed its usage

Cleaned up leftover usings

* Missed one comment

src/Ryujinx.Ava/UI/Helpers/NullableDateTimeConverter.cs

@@ -0,0 +1,38 @@
+using Avalonia.Data.Converters;
+using Avalonia.Markup.Xaml;
+using Ryujinx.Ava.Common.Locale;
+using System;
+using System.Globalization;
+
+namespace Ryujinx.Ava.UI.Helpers
+{
+    internal class NullableDateTimeConverter : MarkupExtension, IValueConverter
+    {
+        private static readonly NullableDateTimeConverter _instance = new();
+
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            if (value == null)
+            {
+                return LocaleManager.Instance[LocaleKeys.Never];
+            }
+
+            if (value is DateTime dateTime)
+            {
+                return dateTime.ToLocalTime().ToString(culture);
+            }
+
+            throw new NotSupportedException();
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            throw new NotSupportedException();
+        }
+
+        public override object ProvideValue(IServiceProvider serviceProvider)
+        {
+            return _instance;
+        }
+    }
+}