Annotate a few AppKit members with ThreadSafe

See comments on this commit:
https://github.com/mono/monomac/commit/2a986b2286adae0695cb26722483ca9b57e0172e

And bug 10604:
https://bugzilla.xamarin.com/show_bug.cgi?id=10604
Merge pull request #101 from spicypixel/hotfix/threadsafe-annotations

Annotate a few AppKit members with ThreadSafe