Added WindowState for mobile platforms.

They always return Normal.

src/Android/Perspex.Android/Platform/SkiaPlatform/MainWindowImpl.cs

@@ -1,5 +1,6 @@
 using Android.Views;
 using Perspex.Android.Platform.Specific;
+using Perspex.Controls;
 using Perspex.Input;
 using Perspex.Platform;
 
@@ -13,6 +14,12 @@ namespace Perspex.Android.Platform.SkiaPlatform
         {
         }
 
+        public WindowState WindowState
+        {
+            get { return WindowState.Normal; }
+            set { }
+        }
+
         protected override void Init()
         {
             base.Init();

src/Android/Perspex.Android/Platform/SkiaPlatform/WindowImpl.cs

@@ -61,6 +61,11 @@ namespace Perspex.Android.Platform.SkiaPlatform
                 _keyboardHelper.HandleEvents = _handleEvents;
             }
         }
+        public WindowState WindowState
+        {
+            get { return WindowState.Normal; }
+            set { }
+        }
 
         public virtual Point GetPerspexPointFromEvent(MotionEvent e) => new Point(e.GetX(), e.GetY());
 

src/iOS/Perspex.iOS/PerspexView.cs

@@ -74,6 +74,11 @@ namespace Perspex.iOS
 
         public IPlatformHandle Handle => PerspexPlatformHandle;
 
+        public WindowState WindowState
+        {
+            get { return WindowState.Normal; }
+            set { }
+        }
 
         public override void LayoutSubviews() => Resized?.Invoke(ClientSize);