Expose OS information to script

The Installer Framework should provide an API to expose the Operation System type, version, and bitness to script. This could be written as a wrapper to the - greatly enhanced - QSysInfo API in Qt5Core / Qt 5.4.

Motivation

The current IFW features a few hooks to get information about the OS:

installer.value("os"): Returns either "win", "x11", or "mac" . The names cannot be updated anymore.

installer.environmentVariable("ProgramFiles(x86)"): Is the current ad-hoc way to check for x32 bit or x64 bit windows.

There's no easy check to e.g. test for the Windows version (say to exclude XP). Older attempts to mitigate tried to enumerate all possible Windows / OS X versions , which is however cumbersome.

API

The exposed API should be just a wrapper for QSysInfo:

sysInfo.buildAbi()
sysInfo.currentCpuArchitecture()
sysInfo.productType()
sysInfo.productVersion()
sysInfo.kernelType()
sysInfo.kernelVersion()
sysInfo.prettyProductName()

Alternative root objects: "system", or "installer.system".