Continuous build

Ports FileData to using FileDataContext instead of global variables.

This integration also required pulling more originally-static functions into the
FileData class, so that they would have access to the `context` private member.

In future, once FileData is switched from glib `g_free`/`g_delete` to C++
`new`/`delete`, it would probably be good to switch `context` from being a
`FileDataContext` pointer to a non-nullable reference that's assigned in the
constructor.