Rework much of options UI

Application/Options/MarginSetter.cs

@@ -0,0 +1,73 @@
+using JetBrains.Annotations;
+using System.Windows;
+using System.Windows.Controls;
+
+namespace FeedCenter.Options;
+
+public class MarginSetter
+{
+    private static Thickness GetLastItemMargin(DependencyObject obj)
+    {
+        return (Thickness) obj.GetValue(LastItemMarginProperty);
+    }
+
+    [UsedImplicitly]
+    public static Thickness GetMargin(DependencyObject obj)
+    {
+        return (Thickness) obj.GetValue(MarginProperty);
+    }
+
+    private static void MarginChangedCallback(object sender, DependencyPropertyChangedEventArgs e)
+    {
+        // Make sure this is put on a panel
+        if (sender is not Panel panel)
+            return;
+
+        // Avoid duplicate registrations
+        panel.Loaded -= OnPanelLoaded;
+        panel.Loaded += OnPanelLoaded;
+
+        if (panel.IsLoaded)
+        {
+            OnPanelLoaded(panel, null);
+        }
+    }
+
+    private static void OnPanelLoaded(object sender, RoutedEventArgs e)
+    {
+        var panel = (Panel) sender;
+
+        // Go over the children and set margin for them:
+        for (var i = 0; i < panel.Children.Count; i++)
+        {
+            var child = panel.Children[i];
+
+            if (child is not FrameworkElement fe)
+                continue;
+
+            var isLastItem = i == panel.Children.Count - 1;
+            fe.Margin = isLastItem ? GetLastItemMargin(panel) : GetMargin(panel);
+        }
+    }
+
+    [UsedImplicitly]
+    public static void SetLastItemMargin(DependencyObject obj, Thickness value)
+    {
+        obj.SetValue(LastItemMarginProperty, value);
+    }
+
+    [UsedImplicitly]
+    public static void SetMargin(DependencyObject obj, Thickness value)
+    {
+        obj.SetValue(MarginProperty, value);
+    }
+
+    // Using a DependencyProperty as the backing store for Margin.  This enables animation, styling, binding, etc...
+    public static readonly DependencyProperty MarginProperty =
+        DependencyProperty.RegisterAttached("Margin", typeof(Thickness), typeof(MarginSetter),
+            new UIPropertyMetadata(new Thickness(), MarginChangedCallback));
+
+    public static readonly DependencyProperty LastItemMarginProperty =
+        DependencyProperty.RegisterAttached("LastItemMargin", typeof(Thickness), typeof(MarginSetter),
+            new UIPropertyMetadata(new Thickness(), MarginChangedCallback));
+}