The vcpkg tool has introduced a proper way to use manifests, https://devblogs.microsoft.com/cppblog/vcpkg-accelerate-your-team-development-environment-with-binary-caching-and-manifests/. This PR replaces the custom text file mechanism with the new manifest approach.
It is planned that vckpg manifests will include the ability to version dependencies in the future. Dependency versions would solve a number of issues that currently require workarounds with the appveyor CI.
Set vcpkg manifest version to 1 to avoid any perception it's related to any release or other version numbering.
