add small github actions script for a windows build

[Bomme]

This is a short working example to play around with Github actions and the Windows runner.
Currently it only builds Eigen lib and successfully configures the build.

I put the Eigen lib in packaging\win32_3rdparty but it seems that this dir was used for the cross-built dependencies. Should we introduce a new dir, e.g. win64?

I need to explicitly enable UTF-8 mode for python because this special character → could not be printed. Maybe there is a nicer solution for this?

[Bomme]

@dbogdanov you can check how the actions build looks like here: https://github.com/Bomme/essentia/actions

I think the actions don't get triggered in this repository until the yml file is on a branch that is specifially mentioned, i.e. either we merge or we put this branch in this list if you want to have Actions enabled:

on:
  push:
    branches: [ master ]