Move MaxClientSize to ITopLevelImpl.

This allows the maximum size calculation constraint to be applied when
an `ItemsPresenter` is in a `Popup`, meaning that all items don't get
materialized when the popup is shown. Fixes #719, hopefully.

src/Avalonia.Controls/Platform/ITopLevelImpl.cs

@@ -22,6 +22,11 @@ namespace Avalonia.Platform
         /// </summary>
         Size ClientSize { get; set; }
 
+        /// <summary>
+        /// Gets the maximum size of a window on the system.
+        /// </summary>
+        Size MaxClientSize { get; }
+
         /// <summary>
         /// Gets the scaling factor for the window.
         /// </summary>

src/Avalonia.Controls/Platform/IWindowImpl.cs

@@ -11,11 +11,6 @@ namespace Avalonia.Platform
     /// </summary>
     public interface IWindowImpl : ITopLevelImpl
     {
-        /// <summary>
-        /// Gets the maximum size of a window on the system.
-        /// </summary>
-        Size MaxClientSize { get; }
-
         /// <summary>
         /// Gets or sets the minimized/maximized state of the window.
         /// </summary>

src/Avalonia.Controls/Presenters/ItemsPresenter.cs

@@ -99,7 +99,7 @@ namespace Avalonia.Controls.Presenters
             // the available size.
             if (availableSize == Size.Infinity && VirtualizationMode != ItemVirtualizationMode.None)
             {
-                var window = VisualRoot as Window;
+                var window = VisualRoot as TopLevel;
 
                 if (window != null)
                 {