Add siteSets

[Rathch]

In the course of the Typo3 update, I converted all constants in site sets. I am happy to contribute this here.

[featdd]

Hi @Rathch,

thank you very much for your contribution, this was asked before and already on my todo list as well.
I was planning to add this for the next major release with the v14 compatibility.

I'll keep your pull request ready, I'll have some vacation in april when the feature freeze is also planned, then I can start to integrate this.

Greetings
Daniel