Allow setting the time instant precisely

[S-Dafarra]

Right now it is possible to change the time instant from the slider and from the shortcuts. With long time periods, the mouse is really hard to use, while the shortcuts are easy to forget, and are not intuitive for a new user.

Qt defines the widget QSpinBox to quickly edit a number. It might be useful to precisely control the visualized time instant without resorting to the mouse or shortcuts

Reference https://doc.qt.io/qtforpython-5/PySide2/QtWidgets/QSpinBox.html

cc @GiulioRomualdi