Extend the auto-detection to find cl.exe on the system, not just in the PATH

Currently Qbs either relies on a toolchainInstallPath being set in a profile or otherwise, or that cl.exe is in the PATH, in order to build with MSVC. Instead of the BinaryProbe we have now, we should extend this to a more specific MSVC compiler probe, which looks in PATH and searches the system/registry for the installed location of the latest MSVC available on the system (if a specific version number is supplied, we could also constrain the search to that version).

This is necessary for "out of the box" building with MSVC and profile:none

A "cpp.preferredCompilerVersion" property might be a good idea as a part of this implementation, since version constraints are typically quite important with MSVC toolchains.