This allows multiple versions (e.g. Debug/Release, x86/x64) of micropython.exe
to co-exist instead and also solves potential problems where msbuild does not
completely rebuild the output and/or pdb files when switching between builds,
which in turn can cause linker errors in dependent projects.

By default exe/map/... files go in windows/build/$(Configuration)$(Platform)

After each build micropython.exe is still copied from the above directory to
the windows directory though, as that is consistent with the other ports and
the test runner by default uses that location as well.

Also rename env.props -> path.props which is a clearer name,
and add ample documentation in the affected build files.

(also see discussion in #1538)

This defaults to 'python' but can be now overridden if needed

Also rename py-version.h to mpversion.h for consistency with mpconfig.h.

Remove some duplication in the code for generating
qstrdefs.generated.h and py-version.h

- Use a single file env.props for defining the main directories used when building.
  env.props resolves the base directory and defines overridable output directories,
  and is used by all other build files.
- Fix the build currently failing, basically because the preprocessing command for generating
  qstrdefs uses different include directories than the build itself does.
  (specifically, qstrdefs.h uses #include "py/mpconfig.h" since the fixes for #1022
  in 51dfcb4b, so we need to use the base directory as include directory, not the py dir itself).
  So define a single variable containing the include directories instead and use it where needed.

This fixes generating the headers casuing complete rebuilds,
even when the headere's content didn't really change.

Extend the windows port so it compiles with the toolchain from Visual Studio 2013