Feature Request: support recurrencies based on scheduled/wait instead of due date

There are tasks, were it is important, that they are done (sometimes also in a timely manner), but not exactly on a specific due date (or where that is not a hard deadline). And sometimes these are recurrent.

In general, I have a clear separation between "hard" due dates, which must not be missed (under normal circumstances), and between "soft" due dates, where missing it by a little bit is not a problem by itself, but e.g. just shifts other stuff. That way, it is easier for me to keep "hard" due dates.

With the current recurrent task support, I am required to use `due` just for technical reasons. So it would be helpful for me if the recurrent tasks could be based of other dates like `scheduled` or `wait`.

One possibility to implement this would be to allow users to specify which is the reference date, like `recurOn:wait`. With that, the implementation could easily be extended to even use dates stored in UDAs.

I am aware that a plugin could possibly replace this functionality in probably good enough. So feel free to close if this feature is not feasible for you to be implemented into TW’s core.

Also, I see following issues which might overlap with mine: #1318