Allow defining context objects as parameter to Promise::then()
Using context objects in PromiseSitter::addPromise()
is intricate because one has to add all dependent objects of the complete promise chain when adding the last promise of the chain.
A better approach could be to allow setting the context objects in Promise::then()
and instead of "destroying" the promise, just prevent the execution of the original callback and reject the promise with a special value when the callback would have been called.
However, I'm not 100% sure this will work. So it needs some investigation first.