background: refactor file loading

This commit moves the code around a bit such that the
caller gets allocated up front and then a file load is either
found or created to attach the caller to.

Functionally, the code is the same, it's just now factored in a way
that will make it easier to fix a bug with cancellation later.

https://bugzilla.gnome.org/show_bug.cgi?id=722149