Need cpp.platformIncludePaths like cpp.platformDefines

As per the discussion here: http://lists.qt-project.org/pipermail/qbs/2012-March/000071.html

There needs to be a way for templates like QmlApp.qbs to avoid directly setting cpp.includePaths - possibly introducing a cpp.platformIncludePaths property like there is already cpp.defines and cpp.platformDefines. (Maybe cpp.templateIncludePaths would be a more accurate name but I guess the platform might well have includes too).