Add FuncReverse() to the Interval system

[kamgha]

Description

This is a feature request:

The interval system in DIRECT has Func(), which gets triggered only when the cursor for the Interval is going forward. However, there is no building block for the reverse case, when playRate is negative. I'm proposing a new FuncReverse() building block for the interval system that would be triggered when the cursor goes backwards (e. g. play_rate=-1.0 or on a new iteration in a loop, or when the cursor is set to before FuncReverse()).

Use Case

Can be used to undo something that has been done through Func(). In general, playRate should be seen as an arbitrary value and hence the feature set and the expected behaviour should not differ whether the playRate is positive or negative.

Being just another building block to the interval system keeps the system flexible and easy to use. Parallel(Func(), FuncReverse()) could be created to have a do/undo at the same position. But this is not always desired.