Fix crash in macdoc

ScrollToVisible is sometimes called on the previous node, and in some cases this previous node does not have a registered wrapper which causes macdoc to crash. This fix adds a check for the node instead of throwing.
Corrected a dead link.
Merge pull request #115 from MichaelJames6/patch-1

Corrected a dead link.
Merge pull request #114 from spicypixel/hotfix/macdoc-scrolltovisible-crash

Fix crash in macdoc
NSApplication.Init: throw if called more than once
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 (...))
maccore: add as a submodule
Makefile: update clean target
Simplify the mac documentation updating
Fix disposing of platform types. Fixes #15375.

The handle field was getting cleared out before we were calling 'release'
on it, causing us to leak every instance of a platform type. Putting the
handle in a local variable before it's cleared out fixes this.