feat: add preview table of altered categories

The implementation changed a lot, each time the customization is submitted, initial categories are restored before applying the new custo.

Changed are also applied if a previous custo existed when the plugin is installed.

The preview table is updated right after a successful form submission. This is actually very hacky - might be a best hack - but it does the job for now.

A cheap Json Validation has been implemented and is actually not that bad.