Add built-in support for .icns format (Apple's Icon format)

[vpenades]

I'm aware that microsoft .Ico format has been implemented and it'll be available in the next major version.

I was wondering if it could be possible to add an implementation for Apple's Icon own format, .icns, which is equivalent in features to microsoft's .Ico format, but requires a totally different implementation.

https://en.wikipedia.org/wiki/Apple_Icon_Image_format

I haven't found any implementation in c#, so I had to develop my own bare bones encoder which is what I need. so in case this goes forward, I can provide the source code.