Use OS-agnostic file separators across whole codebase

[ttton]

As noted in #31 and partially resolved in #47, there is a possible issue with using file path separators that are not interoperable across filesystems. We need to make sure that the library can run on using any filesystem, notably on Windows OS as well as *nix-based systems.

So far the convert:to:learnosity code path is believed to be fixed in #47. There are still some gaps in the codebase, for example in the convert:to:qti code path. This needs to be given similar treatment. Other areas of the code that deal with file paths (in the context of the local execution environment) should be checked as well.

Acceptance Criteria

• Runs on *nix filesystems
• Runs on Windows OS filesystems
• Is fixed in convert:to:qti

Related issue: #31