[Foundation] Move NSAttributedStringDocumentAttributes.ViewMode to shared code.

This property has also been added on macOS (where it didn't exist before), since
both documentation and headers reveal it's available there.

src/Foundation/Enums.cs

@@ -25,7 +25,6 @@ namespace Foundation {
 	}
 
 	// Utility enum, ObjC uses NSString
-	[NoMac]
 	public enum NSDocumentViewMode {
 		Normal,
 		PageLayout

src/Foundation/NSAttributedString.iOS.cs

@@ -41,22 +41,6 @@ namespace Foundation {
 
 	public partial class NSAttributedStringDocumentAttributes : DictionaryContainer {
 #if !MONOMAC && !COREBUILD
-		public NSDocumentViewMode? ViewMode {
-			get {
-				var value = GetInt32Value (NSAttributedStringDocumentAttributeKey.NSViewModeDocumentAttribute);
-				if (value is null)
-					return null;
-				else
-					return (NSDocumentViewMode) value.Value;
-			}
-			set {
-				if (value is null)
-					RemoveValue (NSAttributedStringDocumentAttributeKey.NSViewModeDocumentAttribute);
-				else
-					SetNumberValue (NSAttributedStringDocumentAttributeKey.NSViewModeDocumentAttribute, (int) value.Value);
-			}
-		}
-
 		public bool ReadOnly {
 			get {
 				var value = GetInt32Value (NSAttributedStringDocumentAttributeKey.NSReadOnlyDocumentAttribute);

src/Foundation/NSAttributedStringDocumentAttributes.cs

@@ -174,6 +174,16 @@ namespace Foundation {
 				SetNumberValue (NSAttributedStringDocumentAttributeKey.NSViewZoomDocumentAttribute, value);
 			}
 		}
+
+		public NSDocumentViewMode? ViewMode {
+			get {
+				return (NSDocumentViewMode?) GetInt32Value (NSAttributedStringDocumentAttributeKey.NSViewModeDocumentAttribute);
+			}
+			set {
+				SetNumberValue (NSAttributedStringDocumentAttributeKey.NSViewModeDocumentAttribute, value is null ? null : (int) value.Value);
+			}
+		}
+
 #if !TVOS && !WATCH
 		// documentation is unclear if an NSString or an NSUrl should be used...
 		// but providing an `NSString` throws a `NSInvalidArgumentException Reason: (null) is not a file URL`