Modern UI, Loading screen CanRunMessages

src/GuiApp/FileTime.Avalonia/Converters/BoolInverter.cs

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+using System;
+using System.Globalization;
+using Avalonia.Data.Converters;
+
+namespace FileTime.Avalonia.Converters
+{
+    public class BoolInverter : IValueConverter
+    {
+        public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture) => value is bool b ? !b : value;
+
+        public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}