This is a preliminary step for moving the comparison and
application logic of each section into the dialog.
Essentially, the dialog will pass the duplicate event to each
section, and these sections will modify the event. If the user
presses "Save", the dialog will compare the edited event to
the original event, and apply if they are different.
This also adds the `get_event()` method which will be used
in the future.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/711>
GtkDialog is being deprecated. Instead of using it,
we want to subclass AdwWindow. As part of this change
the content and headerbar are put together in one box.
Using dialog responses to communicate intent to GcalWindow was always
a hack, and as the code is cleaned, it's getting increasingly annoying.
Replace this with a proper 'remove-event' signal ('delete-event' would
conflict with the equally named GTK signal), remove the custom response
types, and cleanup the code.
Hari Rana