TaskTree - change API with regards to done / error handlers

This task addresses in details the 9th point in QTCREATORBUG-28741 master task.

1. Get rid of 3 arg c'tors for CustomTask. Accept only 2 handlers: setup and done.
2. The done handler may take additional "DoneWith doneWith" arg indicating whether the task has finished with success, an error, or whether it was canceled.
3. In order to still support calling only one of done or error handler: add a CallDoneIf enum as a 3rd arg for CustomTask's c'tor:

enum class CallDoneIf {
    SuccessOrError, // default
    Success,
    Error
};

4. Make "DoneWith doneWith" arg optional.
5. Make "const Task &" arg optional.
6. Overload the done handler to take only "DoneWith doneWith" arg, without "const Task &".
7. Make it possible the done handler returns optionally the DoneResult

enum value in order to tweak the success bit.
8. Do the same for group done / error handlers.
9. Unifiy TaskTree::done / TaskTree::errorOccurred signals. Keep just done() signal with the additional "DoneWith doneWith" argument.
10. Add tests for these changes, especially for done handler's tweaks.
11. Update docs.