Since the calendar row widget is becoming an expandable widget,
several APIs were added to the widget:
- `gcal_calendar_row_get_calendar`
- `gcal_calendar_row_add_suffix`
- `gcal_calendar_row_get_first_suffix_child`
- `gcal_calendar_row_add_header_class`
- `gcal_calendar_row_remove_header_class`
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/362>
Currently, the quick add popover implements its own row
widget for each calendar, which duplicates tons of unnecessary
code. This commit streamlines the calendar-combo-row-item row
widget to make it reusable elsewhere.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/362>