[Feature]: Limit index files to 1 per folder, by renaming them instead of creating new ones

## Appreciation
- Thanks so much for making this plug-in. This was exactly what I was looking for

## Problem
- I created the indexed files, but I overlooked that the default prefix was "_Index_of_". So I deleted the prefix "_Index_of_" and pressed the button again and now I have more than 1 index files per folder, when only 1 is needed.

## Feature
- Detect already exisiting index files (maybe by looking for files containing the foldername or looking through each file for a line with "%% Zoottelkeeper... %%")
- if the folder doesn't have one, create one with the new prefix
- and if the folder has one, rename them with the new prefix

## Remarks
- If there is already such a functionality, that I overlooked, I would be happy to hear about it