This emits the `changed` signal, allowing us to filter out a wider
variety of calendars. It also allows us to remove the loops
that needed to re-run every time the user changed the default
calendar or closed the popover.
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>