Currently, GcalEventEditorSection is an interface that doesn't
really do much apart from requiring a property and providing
virtual methods.
This proposed approach turns it into an abstract class, derived
from AdwPreferencesGroup, which gives access to virtual methods
as well as public methods. It comes with the same benefits of
an interface, while sharing a lot of code that were duplicated
in the previous commits.
As a stopgap measure, it overrides several GtkWidget virtual
methods to smoothly transition without causing the commits
to break. These overrides will get removed in a later commit.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/711>
Hari Rana