Keep Cocoa screen list in sync on screen connect.

The backtrace in QTBUG-33012 indicates that the
screen list is getting out of sync on plugging/unplugging
an external monitor. This happens on 10.9 only.

Check if we are about to out of bounds access the
screen list; call updateScreens() which will add the
new screen.

Task-number: QTBUG-33012
Change-Id: Id1acc3baca9e60e7f2a83188d66cdf9d322a582c
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>