Children visibility from Task
testing types
Task::slack
correct test
add slacky task in test cycle
adapt test with at5 + test slack and criticalPath
Tasks are allocated using std::make_shared

It needed to put the constructors public.
Private key pattern is used to ensure that the objects
cannot be constructed directly by clients
cosmetics
corrected backward schedule. Slack is now correct in the tested cases
fake cycle build API for test purpose
add TaskTreeModel first impl

Static column model, cycle is the unique root node
add TaskTreeModel and FakeCycles in pro file
MainWindow::ui is held by unique_ptr
add TaskTreeModel to MainWindow, assigned with FakeCycle to the treeView