[Mono-osx] NSImage imageNamed implemenation

[Pavel Sich]

In the current AppKit implementation, there is an error in NSImage, 
method imageNamed is not marked in the interface (interface API - 
appkit.cs) as [Static], therefore it does not work as expected then.

Fix is very simple:
         [Static]
         [Export ("imageNamed:")]
         NSImage ImageNamed (string name);

Cheers,
Pavel Sich
twitter: @sichy