Allow setting the time instant precisely

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

Start by locating the existing time slider and shortcut handling, then review the referenced Qt QSpinBox documentation. The work is done when users can precisely set the visualized time instant through an editable numeric control without relying on the mouse or shortcuts.

Written by the indexing model from the issue text.