ActionCallback now only has one generics argument that has to implement
IntoLuaMulti, this makes ActionCallback much more flexible as it no
longer always requires two arguments.
This can be very useful if you want to convert a data struct to a lua
table without having to write the boilerplane (however small it may
be).
It also adds the macro on several state structs so they can be
converted to lua in the upcoming ActionCallback refactor.
