Refresh TODO explorer to match other UI elements, refactor grouping t…

…o get rid of duplicated grid control.

Rubberduck.Core/UI/Converters/GroupingToBooleanConverter.cs

@@ -0,0 +1,42 @@
+using System;
+using System.Globalization;
+using System.Windows.Data;
+using Rubberduck.UI.ToDoItems;
+
+namespace Rubberduck.UI.Converters
+{
+    public class ToDoItemGroupingToBooleanConverter : GroupingToBooleanConverter<ToDoItemGrouping> { }
+
+    /// <summary>
+    /// Provides a mutually exclusive binding between an ToDoItemGrouping and a boolean. 
+    /// Note: This is a stateful converter, so each bound control requires its own converter instance.
+    /// </summary>
+    public class GroupingToBooleanConverter<T> : IValueConverter where T : IConvertible, IComparable
+    {
+        private T _state;
+
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            if (!(parameter is T governing) ||
+                !(value is T bound))
+            {
+                return false;
+            }
+
+            _state = bound;
+            return _state.Equals(governing);
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            if (!(parameter is T governing) ||
+                !(value is bool isSet))
+            {
+                return _state;
+            }
+
+            _state = isSet ? governing : _state;
+            return _state;
+        }
+    }
+}