Update Windows instructions to current situation, and include vcpkg as an alternative to MSYS2.

This merge request follows a discussion on gtk-list, see https://mail.gnome.org/archives/gtk-list/2019-March/msg00029.html.

I have updated the Windows download/build/installation guide with instructions on how to use Microsoft's vcpkg packages, which work better in Visual Studio projects. I have also added some more instructions on what needs to be done to get a decent Windows 10 theme and the associated icons. This can still be improved, and I am happy to keep working on this, but already this merge should save many people a lot of time digging around for information.