TYPO3 Core

This change adds the feature that php files in the
extension directory Configuration/TCA/Overrides/
will be executed right before building the cached
TCA. These files must only contain PHP code
that manipulate TCA (like addTCAColumns or
addToAllTCATypes).

The benefit in doing so is that we can move all code
manipulating the TCA that previously was in ext_tables.php
and therefore uncachable, can now be in a defined
location and is only executed when TCA is built
and after that cached.

This will give backend requests quite a performance
boost once extensions start using these files.