File save feature saves to an incompatible path on macOS
Description: After clicking 'Save' for a particular file, it saves in a Windows-like configured path, even though I run on macOS.
Reproduction:
Simply open a New Folder in the app, and a New File within. After saving it, you'll discover New File is saved outside of New Folder, with the name "files\New Folder\New File.json". Its path: "/Users/<user>/Documents/Mathberet/files\New Folder\New File.json"
Expected behavior: File path should have its backslashes reverted.
Screenshots:

- OS: macOS
@zivattias The functions in the app that deal with paths currently handles only windows paths (\\ or \). Some of the functions also have TODO comments suggesting to make the path formatting cross-platform. Addressing this will probably also fix #9.
Proposed a fix here: PR #33