The implementation of the credentials popover has relied on a hack,
and sadly this hack doesn't work anymore.
It's too late for a redesign, so for now, just put it into a dialog
and show it on top of the calendar management dialog.
The color chosen in the new calendar page is currently ignored
when creating a new calendar, as described in #681.
Added a callback similar to that used for the name text entry,
allowing new calendars to be created with the chosen color.
Closes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/681
Georges Basile Stavracas Neto