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When the instance restart, the register method is called
before the Rest server start: we can't fetch initial categories
in register method, server listenning hook is required...
The inconvenient we can't run this on an install or we need
to register another hook when the plugin is installed.

Add better error handling and better logging overall.

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.

Since categories constants modifications are done in memory,
these are all wiped out when the server stop.
This allow to restart the instance without loosing the categories customization.