And make it an enum class

...since it's unused.

How could this build with g++ ?!

This makes it easier to scan a row and extract values to variables,
without having to manually pass the column index.
In the likely event we add/remove columns, this will help prevent
regressions as well.

This might need to be reverted when properly addressing files deletion
(while monitoring filesystem or when reloading)
However, as of now, I'm not confident this respect the object ownership
graph anyway

This is mostly a workaround before a proper architectural fix