utils: Add utility function for creating activate signal and shortcuts

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>

src/utils/gcal-utils.h

@@ -141,4 +141,7 @@ SoupSession *        gcal_create_soup_session                    (void);
 GListModel *         gcal_create_writable_calendars_model        (GcalManager        *manager);
 
 const gchar *        gcal_get_email_from_mailto_uri              (const gchar        *mailto_uri);
+
+guint                gcal_create_activate_signal_and_shortcuts   (GtkWidgetClass *widget_class,
+                                                                  GType           widget_type);
 #endif /* __GCAL_UTILS_H__ */