Add custom file playback to playef()

[Fussmatte]

Changes:

This PR allows users to call custom filenames via the playef() command. For example, playef(mysfx) will play back either sounds/mysfx.ogg or sounds/mysfx.wav. Custom sounds are loaded with the level, and (for now) only the level's own sounds/ folder is read from.

This uses a new function called FILESYSTEM_enumerateAssets() which gets all the files from a specific folder of a custom level's assets folder. This could allow other possibilities for loading custom files (such as graphics) in the future.

Big thanks to @Dav999-v for helping me write this!

Legal Stuff:

By submitting this pull request, I confirm that...

• My changes may be used in a future commercial release of VVVVVV
• I will be credited in a CONTRIBUTORS file and the "GitHub Friends"
  section of the credits for all of said releases, but will NOT be compensated
  for these changes

[Fussmatte]

Worth noting: if you run this version right now, .ogg files cannot be loaded this way. This is due to #900, so once that's fixed it should work with no issues.

[Daaaav]

I just needed FILESYSTEM_enumerateAssets() for #922, so it's now in my PR as well - meaning if #922 gets merged before this one, all changes this makes to FileSystemUtils.[cpp/h] are no longer necessary and need to be undone. Also, I decided to change how it works, so this will also need to be replaced:

    std::vector<std::string> customSounds = FILESYSTEM_enumerateAssets("sounds/");
    for (int i = 0; i < customSounds.size(); i++)
    {
        // use customSounds[i]
    }

by something like this:

    EnumHandle* handle = {};
    const char* item;
    while ((item = FILESYSTEM_enumerateAssets("sounds/", &handle)) != NULL)
    {
        // use item
    }
    FILESYSTEM_freeEnumerate(&handle);

[Fussmatte]

Everything here should be in order now; I've squashed it all to one commit and made sure it's building and functional. @InfoTeddy could you look at this?

[InfoTeddy]

Yes.

[Fussmatte]

So apparently OGG files don't work here after all, even though OGG playback was fixed. Will have to look into this before it can be merged...