Port QMacNativeWidget to Qt 5

Also adds examples/widgets/mac subdir for Mac specific examples,
starting with one for this feature.

Change-Id: I4cc7d84ce3d7562259d6206faa5d6996c2392a3e
Reviewed-by: Liang Qi <liang.qi@digia.com>
Port QMacCocoaViewContainer to Qt 5.

Includes example.

Change-Id: Ifdda5c535d0ec41694712405d921b2c32cb8dfc8
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>