Keep value of registry record between uninstalling and reinstall package

[espenmn (Espen)]

You could probably use a 'setuphandlers' (there is a pre_handler and a post_handler) and set the value (if it does not exist) with:

plone.api.portal.set_registry_record

Also: make sure that the uninstall profile does not remove it.

Generic setup, 'pre-handler'

I am the one who added post_handler to GenericSetup in 2016, see this PR. And this same PR introduces pre_handler, which works exactly the same as post_handler. The order is: Run the pre_handler, if available. Run the import steps. Run the post_handler, if available.