Allow paths from multiple sources, including other operating systems #15
Description
It would be nice to use the same database on any platform. However, we currently use one column to store paths for all operating systems.
Initially, we could just detect Unix/Windows, and store their respective paths in a table separated from variants - though this would mean adding an unique index to the variants table, and removing the id column.
This approach allows supporting multiple systems which mirror the same books via different paths - for this use case, we give each system an unique UUID stored in the settings file, and we select paths belonging to that UUID, stored in the table for the Unix/Windows case. This can also be extended in the future to new systems or alternative path formats (eg. URLs).