QMacStyle: Fix focus frame rendering in Yosemite

Using the HITheme API would result in the frame's right edge
to be missing.

Instead, we use the recommended technique to draw the focus
ring around a custom NSCell. (See
https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/ControlCell/Articles/ManipulateCellControl.html)

Backport of 769af6689337285927a2637b184149f688332542 from Qt5/qtbase

Change-Id: I8f3992edc065e14632ec4e473732b30bcf4c1937
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>