Paths more than one dir deep are 'Unused files' on Windows and save inconsistently

If a file is saved more than one dir deep, Inky running on Windows sorts it under 'Unused files'. Well, actually it both includes and doesn't include the file, and this seems to cause all sorts of strange saving issues.

For example:

```
INCLUDE names_db.ink
INCLUDE spaces/pub_lounge.ink
INCLUDE spaces/pub_lounge/yvonne.ink
INCLUDE spaces/pub_lounge/jukebox.ink
INCLUDE spaces/pub_corridor.ink
INCLUDE spaces/pub_womens.ink
INCLUDE spaces/pub_mens.ink
INCLUDE spaces/debug_gym.ink
INCLUDE state.ink
INCLUDE spaces/pub_lounge/something/blah.ink
```

in the main file shows as:

This was driving me crazy (my beautiful writing wasn't saving!), so I've tracked down the issue and will create a PR with the fix imminently.