[appkit] Add NSImage lazy initialization

We were missing initByReferencingFile: which initializes
the image object lazily.

The non working use case was:

var iconFile = NSBundle.MainBundle.PathForResource ("AppIcons", "icns");
NSApplication.SharedApplication.ApplicationIconImage = new NSImage (iconFile);

In this example, the constructor is calling initWithContentsOfFile: which will
not load the right icon from the .icns file.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=40349.

src/AppKit/NSImage.cs

@@ -88,6 +88,14 @@ public static NSImage FromStream (System.IO.Stream stream)
 			}
 		}
 
+		public NSImage (string fileName, bool lazy)
+		{
+			if (lazy)
+				Handle = InitByReferencingFile (fileName);
+			else
+				Handle = InitWithContentsOfFile (fileName);
+		}
+
 		public NSImage (NSData data, bool ignoresOrientation)
 		{
 			if (ignoresOrientation) {

src/appkit.cs

@@ -8183,15 +8183,19 @@ public partial interface NSImage : NSCoding, NSCopying, NSSecureCoding, NSPasteb
 		[Export ("initWithContentsOfURL:")]
 		IntPtr Constructor (NSUrl url);
 
-		//[Export ("initByReferencingFile:")]
-		//IntPtr Constructor (string fileName);
 		//[Export ("initByReferencingURL:")]
 		//IntPtr Constructor (NSUrl url);
 
 		// FIXME: need IconRec
 		//[Export ("initWithIconRef:")]
 		//IntPtr Constructor (IconRef iconRef);
 
+		[Sealed, Export ("initWithContentsOfFile:"), Internal]
+		IntPtr InitWithContentsOfFile (string fileName);
+
+		[Export ("initByReferencingFile:"), Internal]
+		IntPtr InitByReferencingFile (string name);
+
 		[Export ("initWithPasteboard:")]
 		IntPtr Constructor (NSPasteboard pasteboard);