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.
This will be used by the pages of the calendar management dialog
to let it know they are actual pages, to offer an activation
mechanism, and to communicate with the dialog when they want to
switch to another page.
Hari Rana