OverflowMode.Always bug fix (#2540)

Author: MichelMichels

MaterialDesignThemes.Wpf/Converters/ToolBarOverflowButtonVisibilityConverter.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Globalization;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Data;
+
+namespace MaterialDesignThemes.Wpf.Converters
+{
+    public class ToolBarOverflowButtonVisibilityConverter : IMultiValueConverter
+    {   
+        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
+        {
+            var overflowMode = (OverflowMode)values[0];
+            var hasOverflowItems = (bool)values[1];
+
+            if (overflowMode == OverflowMode.AsNeeded && hasOverflowItems)
+            {
+                return Visibility.Visible;
+            }
+            else
+            {
+                return overflowMode switch
+                {
+                    OverflowMode.Always => Visibility.Visible,
+                    OverflowMode.Never => Visibility.Hidden,
+                    _ => Visibility.Hidden,
+                };      
+            }           
+        }
+        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture) => throw new NotImplementedException();
+    }
+}