event-editor/event-editor-section: Turn it into an abstract class

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>

src/gui/event-editor/gcal-event-editor-section.h

@@ -20,41 +20,37 @@
 
 #pragma once
 
-#include "gcal-event.h"
-
 #include <adwaita.h>
 
+#include "gcal-event.h"
+
 G_BEGIN_DECLS
 
-#define GCAL_TYPE_EVENT_EDITOR_SECTION (gcal_event_editor_section_get_type ())
+#define GCAL_TYPE_EVENT_EDITOR_SECTION (gcal_event_editor_section_get_type())
 
-G_DECLARE_INTERFACE (GcalEventEditorSection, gcal_event_editor_section, GCAL, EVENT_EDITOR_SECTION, AdwPreferencesGroup)
+G_DECLARE_DERIVABLE_TYPE (GcalEventEditorSection, gcal_event_editor_section, GCAL, EVENT_EDITOR_SECTION, AdwPreferencesGroup)
 
-typedef enum
+struct _GcalEventEditorSectionClass
 {
-  GCAL_EVENT_EDITOR_FLAG_NONE      = 0,
-  GCAL_EVENT_EDITOR_FLAG_NEW_EVENT = 1 << 0,
-} GcalEventEditorFlags;
+  AdwPreferencesGroupClass parent_class;
 
-struct _GcalEventEditorSectionInterface
-{
-  GTypeInterface parent;
-
-  void               (*set_event)                                (GcalEventEditorSection *self,
-                                                                  GcalEvent              *event,
-                                                                  GcalEventEditorFlags    flags);
-
-  void               (*apply)                                    (GcalEventEditorSection *self);
-
-  gboolean           (*changed)                                  (GcalEventEditorSection *self);
+  /**
+   * GcalEventEditorSectionClass::event_set_cb:
+   * @self: a #GcalEventEditorSection
+   * @event: (nullable) (transfer none): a #GcalEvent
+   *
+   * Invoked when a new event has been set.
+   *
+   * Subclasses must implement this virtual method and update its children
+   * to reflect the new event.
+   */
+  void (* event_set_cb) (GcalEventEditorSection *self,
+                         GcalEvent              *event);
 };
 
-void                 gcal_event_editor_section_set_event         (GcalEventEditorSection *self,
-                                                                  GcalEvent              *event,
-                                                                  GcalEventEditorFlags    flags);
-
-void                 gcal_event_editor_section_apply             (GcalEventEditorSection *self);
-
-gboolean             gcal_event_editor_section_changed           (GcalEventEditorSection *self);
+GcalEvent *gcal_event_editor_section_get_event  (GcalEventEditorSection *self);
+GtkWidget *gcal_event_editor_section_get_dialog (GcalEventEditorSection *self);
+void       gcal_event_editor_section_set_dialog (GcalEventEditorSection *self,
+                                                 GtkWidget              *dialog);
 
 G_END_DECLS