The current way of adding activation shortcuts is cumbersome, as it is a for-loop pasted in the middle of class initializers. The utility function addresses this problem by simply allowing us to declaratively add these shortcuts with minimal effort. Furthermore, the implementation is also simpler, thanks to the `gtk_widget_class_add_binding_signal()` wrapper function. Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/747>