Cocoa: Fix fonts on retina displays.

Cocoa: Fix fonts on retina displays.

Hardcode logical dpi to 72 again. NSDeviceResolution
gives us the physical dpi (144) and results in double-
sized fonts in Qt. QPlatformScreen does not currently
have a physicalDpi virtual, perhaps this can be added
later on.

Unfortunately the usefulness of a per-screen correct
DPI metric seems questionable to me:
1) The value returned by the system is not correct,
   pixels per inch on the rMBP is around 220.
2) Qt always uses the dpi for the main screen, via
   qt_defaltDpiX/Y.

Change-Id: Ia35804be62ee7f1c623bad854f65d744dc9075d4
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>