Due to some languages using the "nplurals=1; plural=0" plural form,
and the string already in use in src/gui/calendar-management/gcal-new-calendar-page.c
with g_dngettext, add context to the other "Add Calendar" strings to fix the problem.
AFAIK, other locales (confirmed: be_BY, fa_IR, id_ID, kk_KZ, ko_KR, my_MM, zh_CN)
are also affected by this issue.
Move the add button from the headerbar into the dialog, make it
a pill button, and update the button label with the number of
calendars that will be added.
The interfacing that GcalCalendarManagementDialog had is somewhat
similar to AdwNavigationView, with pages and signaling basically
matching the same semantics.
We can save some code by switching to AdwNavigationView then, as
the heavy-lifting of page management is done there now.
The headerbar of the dialog is no more; instead, each page has its
own headerbar that automatically gets a back button.
Page activation and deactivation works essentially the same, but
uses AdwNavigationView signals to manage that.
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
