Cocoa: Work around faulty screen list on startup

The NSScreen API has been observed to a return a
screen list with one mirrored, non-primary screen
when Qt is running as a startup item. Always use
the screen if there's only one screen in the list.

Change-Id: I721e25bb7595599287b97f6528e04060ce5da6c1
Task-id: QTBUG-37878
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Doc: corrected autolink errors Qjsonvalue

Task-number: QTBUG-40362
Change-Id: I2ae782ea606e8ee55ee0fcfbbb29db3216b0574b
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>