Add calendar control

Closes #101

src/Spectre.Console/Internal/Collections/ListWithCallback.cs

@@ -0,0 +1,88 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+namespace Spectre.Console.Internal.Collections
+{
+    internal sealed class ListWithCallback<T> : IList<T>
+    {
+        private readonly List<T> _list;
+        private readonly Action _callback;
+
+        public T this[int index]
+        {
+            get => _list[index];
+            set => _list[index] = value;
+        }
+
+        public int Count => _list.Count;
+        public bool IsReadOnly => false;
+
+        public ListWithCallback(Action callback)
+        {
+            _list = new List<T>();
+            _callback = callback ?? throw new ArgumentNullException(nameof(callback));
+        }
+
+        public void Add(T item)
+        {
+            _list.Add(item);
+            _callback();
+        }
+
+        public void Clear()
+        {
+            _list.Clear();
+            _callback();
+        }
+
+        public bool Contains(T item)
+        {
+            return _list.Contains(item);
+        }
+
+        public void CopyTo(T[] array, int arrayIndex)
+        {
+            _list.CopyTo(array, arrayIndex);
+            _callback();
+        }
+
+        public IEnumerator<T> GetEnumerator()
+        {
+            return _list.GetEnumerator();
+        }
+
+        public int IndexOf(T item)
+        {
+            return _list.IndexOf(item);
+        }
+
+        public void Insert(int index, T item)
+        {
+            _list.Insert(index, item);
+            _callback();
+        }
+
+        public bool Remove(T item)
+        {
+            var result = _list.Remove(item);
+            if (result)
+            {
+                _callback();
+            }
+
+            return result;
+        }
+
+        public void RemoveAt(int index)
+        {
+            _list.RemoveAt(index);
+            _callback();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
+    }
+}

src/Spectre.Console/Internal/Extensions/DayOfWeekExtensions.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Globalization;
+
+namespace Spectre.Console.Internal
+{
+    internal static class DayOfWeekExtensions
+    {
+        public static string GetAbbreviatedDayName(this DayOfWeek day, CultureInfo culture)
+        {
+            culture ??= CultureInfo.InvariantCulture;
+            var name = culture.DateTimeFormat.GetAbbreviatedDayName(day);
+
+            if (name.Length > 0 && char.IsLower(name[0]))
+            {
+                name = string.Format(culture, "{0}{1}", char.ToUpper(name[0], culture), name.Substring(1));
+            }
+
+            return name;
+        }
+
+        public static DayOfWeek GetNextWeekDay(this DayOfWeek day)
+        {
+            var next = (int)day + 1;
+            if (next > (int)DayOfWeek.Saturday)
+            {
+                return DayOfWeek.Sunday;
+            }
+
+            return (DayOfWeek)next;
+        }
+    }
+}