Added custom sort logic for right window files

[AndreaDuina]

Currently in the right window of plottr-monitr python files appear first.
This is very inconvenient because they are very rarely checked and occupy a lot of space.

Images should appear first instead, such that users don't have to scroll down every time they want to check a measurement.

I added a very simple custom sort logic, which enforces the following order:

1. Images
2. Folder path
3. Param dict
4. Anything else
5. Python files

[marcosfrenkel]

@AndreaDuina Thank you for the contribution, I haven't been able to test it locally just yet, but it seems that the check is an easy fix, simply specify what the return value is for the sort function and that should be it.

[AndreaDuina]

Should be fine now, let me know!

There are errors for higher python versions due to a missing type definition in utils/num.py, which I haven't touched, so I'm assuming you're already aware of that