Setting a save path on MacOS does not include a trailing '/'

[remutro]

Describe the bug
Setting a save path on MacOS does not include a trailing '/', resulting in new folders being created sue to the resulting path being used. This works properly on Windows as well as on Linux (the trailing '/' is included when writing to the settings.bml file). We should validate that all paths in Settings > Paths work as expected while investigating this issue. Assumption is the file picker on Mac doesn't include the trailing slash, so we likely just need to check the returned path in all cases and add the slash ourselves if missing.

To Reproduce
Steps to reproduce the behavior:

1. On a Mac, open Settings > Paths
2. Select a folder for saves
3. Play a game where a save file will be generated
4. Check the file/folder where the save is actually created

Expected behavior
Files are saved in the selected folder properly, trailing slash included on user defined paths in settings.bml file.