I'm trying to find anything desribing the process and currently avaible features (especially if window tabbing will work with QML decorations) but I can't find anything. Any hints?

The QML API for decorations is not really stable yet. That's why it is not documented at all. If you want to do one, I recommend to have a look at the Plastik decoration which provides more or less all available features - be aware that the API indeed changed in KWin/5. And no, window tabbing is not (yet) supported.

Oh, right. Thank you for the anwer. Hopefully window tabbing will be supported soon. I really like this feature