TCA column type `grid`

[witrin]

The relation between container and their items must be defined by a new TCA column type as well as the data set should use processedTca.columns.<grid column> instead of customData.tx_grid to allow multiple grid columns in a container. See also https://gist.github.com/NamelessCoder/5a0e4e455810819ac78480090f6cd0c5.