TaskTree - prepare much more tests / examples

Some example usages:

1. Creating an object on heap as a result of finishing one task and reusing it in another, sequential, as e.g. an input data. The dynamically created object should always be stored with std::shared_ptr, otherwise it may leak when the tree stops spontaneously. It should be stored inside the custom storage object.
2. Show, that's it possible to create and reuse QObject subclasses (even QWidget subclasses) inside the Storage object. That's not obvious!
3. Add an example utilizing TaskTree for animations / QTimeLine.
4. Add a test that executes the nested TaskTree from withing the outer TaskTree's handler. That isn't strictly the misuse of the API, but it's not the best idea anyway. Place a common storage in the outer and the nested tree. Check, whether the activation of the storage inside the nested tree works OK.