OSX: Fix showing window with no specified size.

A Window without a `Width`/`Height` specified was not getting shown on OSX since 1f8b909257. Set the content view in the constructor to fix this.

native/Avalonia.Native/src/OSX/window.mm

@@ -52,6 +52,7 @@ public:
         [Window setBackingType:NSBackingStoreBuffered];
         
         [Window setOpaque:false];
+        [Window setContentView: StandardContainer];
     }
     
     virtual HRESULT ObtainNSWindowHandle(void** ret) override
@@ -124,7 +125,6 @@ public:
             SetPosition(lastPositionSet);
             UpdateStyle();
             
-            [Window setContentView: StandardContainer];
             [Window setTitle:_lastTitle];
             
             if(ShouldTakeFocusOnShow() && activate)