Currently TCA changes of the category API are applied on every request
after the extTables files is included. This makes it impossible to
override the changes by third party extensions and also we do not
benefit from Caching.
Instead we use the introduced signal and apply the changes right before
With this change we have a clear separation of
registering category field additions (in ext_localconf.php) and
applying the TCA changes while building the TCA cache.