implement max client size and top most.

src/Avalonia.Native.OSX/window.mm

@@ -44,6 +44,13 @@ public:
         return S_OK;
     }
     
+    virtual HRESULT SetTopMost (bool value)
+    {
+        [Window setLevel: value ? NSFloatingWindowLevel : NSNormalWindowLevel];
+        
+        return S_OK;
+    }
+    
     virtual HRESULT Close()
     {
         [Window close];
@@ -60,6 +67,19 @@ public:
         return S_OK;
     }
     
+    virtual HRESULT GetMaxClientSize(AvnSize* ret)
+    {
+        if(ret == nullptr)
+            return E_POINTER;
+        
+        auto size = [NSScreen.screens objectAtIndex:0].frame.size;
+        
+        ret->Height = size.height;
+        ret->Width = size.width;
+        
+        return S_OK;
+    }
+    
     virtual HRESULT GetScaling (double* ret)
     {
         if(ret == nullptr)

src/Avalonia.Native/Helpers.cs

@@ -14,5 +14,10 @@ namespace Avalonia.Native
         {
             return new AvnPoint { X = pt.X, Y = pt.Y };
         }
+
+        public static Size ToAvaloniaSize (this AvnSize size)
+        {
+            return new Size(size.Width, size.Height);
+        }
     }
 }

src/Avalonia.Native/WindowImplBase.cs

@@ -190,9 +190,17 @@ namespace Avalonia.Native
             _native.BeginMoveDrag();
         }
 
-        #region Stubs
+        public Size MaxClientSize => _native.GetMaxClientSize().ToAvaloniaSize();
+
+        public void SetTopmost(bool value)
+        {
+            _native.SetTopMost(value);
+        }
+
         public double Scaling => _native.GetScaling();
 
+        #region Stubs
+
         public Action<Point> PositionChanged { get; set; }
         public Action Deactivated { get; set; }
         public Action Activated { get; set; }
@@ -201,16 +209,11 @@ namespace Avalonia.Native
         Action<double> ScalingChanged { get; set; }
         public IPlatformHandle Handle => new PlatformHandle(IntPtr.Zero, "NOT SUPPORTED");
 
-        public Size MaxClientSize => new Size(1600, 900);
 
         public IScreenImpl Screen => new ScreenImpl();
 
         Action<double> ITopLevelImpl.ScalingChanged { get; set; }
 
-        public void SetTopmost(bool value)
-        {
-        }
-
         public void SetMinMaxSize(Size minSize, Size maxSize)
         {
         }

src/headers/avalonia-native.h

@@ -73,6 +73,7 @@ AVNCOM(IAvnWindowBase, 02) : virtual IUnknown
     virtual HRESULT Hide () = 0;
     virtual HRESULT Close() = 0;
     virtual HRESULT GetClientSize(AvnSize*ret) = 0;
+    virtual HRESULT GetMaxClientSize(AvnSize* ret) = 0;
     virtual HRESULT GetScaling(double*ret)=0;
     virtual HRESULT Resize(double width, double height) = 0;
     virtual void Invalidate (AvnRect rect) = 0;
@@ -81,6 +82,8 @@ AVNCOM(IAvnWindowBase, 02) : virtual IUnknown
     virtual void SetPosition (AvnPoint point) = 0;
     virtual HRESULT PointToClient (AvnPoint point, AvnPoint*ret) = 0;
     virtual HRESULT PointToScreen (AvnPoint point, AvnPoint*ret) = 0;
+    virtual HRESULT SetTopMost (bool value) = 0;
+    
 };
 
 AVNCOM(IAvnPopup, 03) : virtual IAvnWindowBase