NSTextAttachmentCell: add NSImage ctor, allow instantiating

There were two methods marked [Abstract] which was incorrect.
As such the whole NSTextAttachmentCell class was generated
as abstract, meaning it was only usable if subclassed, which
is not the case in Objective-C. As in Objective-C, the following
is now supported: new NSTextAttachmentCell (new NSImage (...))