GTK3 Windows build

[Cas]

With the move to a GTK3 UI we need a way to easily create the Windows package.

I am not sure what the best route is going to be but we need something that can always get the latest versions of GTK3 as we had to use outdated PyGTK versions as no other builds existed.

It would be good to drop the usage of MSVC and go with msys2 but there are hurdles to doing that with regards to Twisted and pywin32 (see #3181). These are the steps for another GTK3 project building with msys2:

​https://www.gramps-project.org/wiki/index.php/Gramps_for_Windows_with_MSYS2

Alternatively we continue with MSVC and roll our own GTK3 builds:

​https://github.com/wingtk/gvsbuild

We will also need to move from bbfreeze to most likely pyinstaller but that is further down the road

Help or suggestions with this would be appreciated as primary development is on Ubuntu.