OS X: Enable Retina support when using Qt 5

Set the NSHighResolutionCapable property to true, to get smooth fonts
etc. on Retina screens.

Change-Id: Ia708723f60a619606204c242dfc2711ed6a1e46c
Reviewed-on: https://codereview.kdab.com/14776
Reviewed-by: Christoph Schleifenbaum <christoph@kdab.com>
Reviewed-by: Frank Osterfeld <frank.osterfeld@kdab.com>