Use Natural Sorting in the file dialogs

[TheRealCatherine]

Right now you get things like:

1-SomeLevel.json
10-SomeOtherLevel.json
2-Level2.json
3-Level3.json
...

because the filenames are alphabetically instead of naturally sorted. There is lots of old advice on how to do this floating around using P/Invoke and a Windows XP-era DLL. This is not how we should do it - we need to find a cross-platform and modern solution.

[TheRealCatherine]

Note: file dialogs are under 3rdParty/FileBrowser (and/or 3rdParty/GraceGames) and opened from UIManager which then calls methods in Serializer