calendar-management: Port to AdwNavigationView

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.