Improve Qt-in-namespace implementation in the Cocoa platform plugin

Objective-C++ does not support namespaces. In order to make this work,
we decorate each symbol with suffix using the macro QT_MANGLE_NAMESPACE.
However, with such a technic each public symbol needs to be explicitly exported through
this macro. The use of @compatibility_alias allows to define alias to use it
automatically and transparently.

Task-number: QTBUG-23946

Change-Id: Id521b8160bab126fda40a9d960277b1c04cc8b66
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>