Refactor: Standard filesystem management for input.

[Symbiomatrix]

I do not see any reason why input should be loaded directly in the nodes rather than call the existing get filenames system. I have a mod which allows custom sorting and it's awkward having to override these classes as well.

Also, I'm not sure why files aren't limited to just image / video file extensions? At the very least loadimage should constrain these. But since this is a refactor I've left it as is (wrote a few common types I can remember in a comment).
Edit: Christian addressed this point in his pr, I've altered the code to fit.

[christian-byrne]

@Symbiomatrix Can you take a look at #7573 as an alternative solution?

[Symbiomatrix]

@christian-byrne That's actually unrelated to this PR - still collects and sorts the files directly rather than rely on get method.
But as far as my second comment, it's a good solution, which I see is already in use in LoadAudio.
If we simply run this after get_filenames, would be fine by me.