Add UnregisterClass method

Closes #451

Author: AArnott

src/User32/User32.cs

@@ -1687,6 +1687,27 @@ public static extern unsafe int QueryDisplayConfig(
         [return: MarshalAs(UnmanagedType.U2)]
         public static extern short RegisterClassEx(ref WNDCLASSEX lpwcx);
 
+        /// <summary>
+        /// Unregisters a window class, freeing the memory required for the class.
+        /// </summary>
+        /// <param name="lpClassName">
+        /// A null-terminated string or a class atom. If lpClassName is a string, it specifies the window class name.
+        /// This class name must have been registered by a previous call to the <see cref="RegisterClass"/> or <see cref="RegisterClassEx"/> function.
+        /// System classes, such as dialog box controls, cannot be unregistered.
+        /// If this parameter is an atom, it must be a class atom created by a previous call to the <see cref="RegisterClass"/> or <see cref="RegisterClassEx"/> function.
+        /// The atom must be in the low-order word of lpClassName; the high-order word must be zero.
+        /// </param>
+        /// <param name="hInstance">A handle to the instance of the module that created the class.</param>
+        /// <returns>
+        /// If the function succeeds, the return value is nonzero.
+        /// If the class could not be found or if a window still exists that was created with the class, the return value is zero. To get extended error information, call <see cref="Marshal.GetLastWin32Error"/>.
+        /// </returns>
+        [DllImport(nameof(User32), SetLastError = true, CharSet = CharSet.Unicode)]
+        [return: MarshalAs(UnmanagedType.Bool)]
+        public static extern bool UnregisterClass(
+            string lpClassName,
+            IntPtr hInstance);
+
         /// <summary>
         /// Retrieves the name of the format from the clipboard.
         /// </summary>