Support debug and release at the same time in an Xcode project

Support having two build targets at the same time allowing switching between debug and release, instead of having to regenerate the project file using qmake "CONFIG+=debug" -spec macx-xcode

NOTES**
The XCode concept of Active Build Configurations is conceptually different to the QMake idea of build targets. So implementing this is not feasible at the moment.

The Xcode Active Build Configurations settings are for modifying xcode configurations, compiler flags and similar build options. Unlike Visual Studio, XCode does not allow for the selection of specific library files based on if debug or release build configurations are selected. The QMake debug and release settings control which library files are linked to the executable.

It is currently not possible to set files in XCode configuration settings from the QMake generated xcode project file. The way the libraries are linked in the "Frameworks & Libraries" phase in the XCode build system.

Furthermore The selected "Active Build Configuration" is stored in a .pbxuser file, this file is generated by xcode on first load so is not created by qmake.

Due to the evolution of Xcode this may be possible in future versions, but currently there is not enough interest to justify the development investment by Qt in the current make tool, QMake.