Implemented gif exportation and added some style (again)

GifResizer/Views/Converter/VisibleWhenNullConverter.cs

@@ -0,0 +1,21 @@
+using System.Globalization;
+using System.Windows;
+using System.Windows.Data;
+
+namespace GifResizer.Views.Converter;
+
+public class VisibleWhenNullConverter : IValueConverter
+{
+    public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        bool invert = parameter?.ToString() == "invert";
+        bool isVisible = invert ? value != null : value == null;
+        
+        return isVisible ? Visibility.Visible : Visibility.Collapsed;
+    }
+
+    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        throw new NotImplementedException();
+    }
+}