This now takes a regular overloaded function and the visiting is done in
`receive_messages()` itself. This way we can use templates to ensure
that the return type is correct. Otherwise auto will cause issues in the
future when we want to return multiple concrete types from a function
that takes a single variant. The alternative would be both receiving a
variant as a parameter and then returning another variant as a result,
but that is much less type safe.
And add separate implementations for the native plugin and the Wine
plugin host. This way we can easily allow the native host to do
callbacks without having to manage a load of lambdas.
Robbert van der Helm