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

Qt supports wrapping all library code in a C++ namespace using the -qtnamespace configure option. Objective-C++ does not support namespaces.

In order to make this work we decorate Qt symbol usage with macros:

QT_MANGLE_NAMESPACE(QCocoaMenuLoader)
QT_PREPEND_NAMESPACE(QFileDialogOptions::FileMode)

However, the Objective-C has a well-hidden feature that might help us: @compatibility_alias. See http://developer.apple.com/library/mac/#documentation/DeveloperTools/gcc-4.0.1/gcc/compatibility_005falias.html

Investigate if and where @compatibility_alias can be used instead of the macros.