Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/711>
This is a prerequisite to make GcalEventEditorSection require inheriting AdwPreferencesGroup. Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/781>
Move all widgetry, logic, and setup to GcalReminderSection.