Allow another ledger file for settings/setup

My use-case:
Right now I have a single `ledger.dat`on my phone that I sync via `syncthing` between my phone and my computer (where all my other ledger files are). I read this ledger file and compare it with my other `ledger.dat` files on the computer and see which entries have been added.

Right now cone only supports one file, which means I have my accounts at the top of the file and the entries after. This become a bit of work whenever I edit my accounts file on the laptop because I can't just sync it back to cone, I have to manually modify the file for the phone as well.

If cone were to support another file to open and use as a set up, it would help me quite a bit to make it a bit less painful to sync my accounts back from the computer to the phone.

No files or tests are named. Start by tracing how the app currently loads the single ledger.dat file and applies its accounts or setup data; done means a separate setup file can be opened and used while the phone ledger remains independently syncable.

Written by the indexing model from the issue text.