[appkit] Add NSImage lazy initialization

[VincentDondain]

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);

Here the constructor is calling initWithContentsOfFile: which will
not load the right icon from the .icns file.

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

[rolfbjarne]

I think it's better to bind the two methods internally (in appkit.cs), and then just chose either depending on the lazy parameter.

It's easy to get manual binding wrong (and in fact I believe you have a leak in your code).

[spouliot]

yes, better to use [Internal] bindings for the reasons @rolfbjarne mentioned and also since the [Export] attribute is used by other tools (e.g. documentation, rosetta, xtro tests).

[VincentDondain]

Alright, I updated the code to be a simple constructor taking a bool instead of a static method.

The story is that I changed my mind while writing the static method initially to make it use the boolean. The plan, as described in https://bugzilla.xamarin.com/show_bug.cgi?id=40349, defined by Chris, was to get 2 static methods (therefore, requiring manual bindings).

Thanks for the feedback guys.

[rolfbjarne]

This won't work, now you have two managed objects using the same native handle.

This means the moment the GC collects one of them, the other will point to random memory, and crash when used.

My suggestion here would be to add another interna binding for this overload, and use that instead (like you did for InitByReferencingFile).

[VincentDondain]

Closing since the changes from my old private fork can't make it into the PR since the repo has been open sourced.