Fix API not supported on Catalina (#830)

NSTableView.FullWidth is only explicitly supported on Big Sur
and later, so don't set it for older versions of macOS.
For older versions of macOS, FullWidth is apparently the default
so doesn't need to be specified.

This updates the layout UI tweaks included in
https://github.com/xamarin/Xamarin.PropertyEditing/pull/817.

Fixes AB#1776438s

Xamarin.PropertyEditing.Mac/MacSystemInformation.cs

@@ -0,0 +1,48 @@
+using System;
+
+namespace Xamarin.PropertyEditing.Mac
+{
+	public class MacSystemInformation
+	{
+		public static readonly Version Ventura = new Version (13, 0);
+		public static readonly Version Monterey = new Version (12, 0);
+		public static readonly Version BigSur = new Version (11, 0);
+		public static readonly Version Catalina = new Version (10, 15);
+		public static readonly Version Mojave = new Version (10, 14);
+		public static readonly Version HighSierra = new Version (10, 13);
+		public static readonly Version Sierra = new Version (10, 12);
+		public static readonly Version ElCapitan = new Version (10, 11);
+		public static readonly Version Yosemite = new Version (10, 10);
+		public static readonly Version Mavericks = new Version (10, 9);
+		public static readonly Version MountainLion = new Version (10, 8);
+		public static readonly Version Lion = new Version (10, 7);
+		public static readonly Version SnowLeopard = new Version (10, 6);
+		static Version version;
+
+		[System.Runtime.InteropServices.DllImport ("/System/Library/Frameworks/Carbon.framework/Versions/Current/Carbon")]
+		static extern int Gestalt (int selector, out int result);
+
+		//TODO: there are other gestalt selectors that return info we might want to display
+		//mac API for obtaining info about the system
+		static int Gestalt (string selector)
+		{
+			System.Diagnostics.Debug.Assert (selector != null && selector.Length == 4);
+			int cc = selector[3] | (selector[2] << 8) | (selector[1] << 16) | (selector[0] << 24);
+			int result;
+			int ret = Gestalt (cc, out result);
+			if (ret != 0)
+				throw new Exception (string.Format ("Error reading gestalt for selector '{0}': {1}", selector, ret));
+			return result;
+		}
+
+		static MacSystemInformation ()
+		{
+			version = new Version (Gestalt ("sys1"), Gestalt ("sys2"), Gestalt ("sys3"));
+		}
+
+		public static Version OsVersion
+		{
+			get { return version; }
+		}
+	}
+}

Xamarin.PropertyEditing.Mac/PropertyList.cs

@@ -18,13 +18,16 @@ namespace Xamarin.PropertyEditing.Mac
 			this.propertyTable = new FirstResponderOutlineView {
 				AccessibilityEnabled = true,
 				AccessibilityTitle = Properties.Resources.AccessibilityPropertyTable,
-				Style = NSTableViewStyle.FullWidth,
 				IndentationPerLevel = 0,
 				SelectionHighlightStyle = NSTableViewSelectionHighlightStyle.None,
 				HeaderView = null,
 				IntercellSpacing = new CGSize (0, 0)
 			};
 
+			// NSTableViewStyle.FullWidth is only supported on macOS 11.0 and later
+			if (MacSystemInformation.OsVersion >= MacSystemInformation.BigSur)
+				this.propertyTable.Style = NSTableViewStyle.FullWidth;
+
 			var propertyEditors = new NSTableColumn (PropertyEditorColId);
 			this.propertyTable.AddColumn (propertyEditors);