event-editor-dialog: Use AdwWindow instead of GtkDialog

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.

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

@@ -19,6 +19,8 @@
 
 #pragma once
 
+#include <adwaita.h>
+
 #include "gcal-event.h"
 #include "gcal-manager.h"
 
@@ -26,7 +28,7 @@
 G_BEGIN_DECLS
 
 #define GCAL_TYPE_EVENT_EDITOR_DIALOG (gcal_event_editor_dialog_get_type ())
-G_DECLARE_FINAL_TYPE (GcalEventEditorDialog, gcal_event_editor_dialog, GCAL, EVENT_EDITOR_DIALOG, GtkDialog);
+G_DECLARE_FINAL_TYPE (GcalEventEditorDialog, gcal_event_editor_dialog, GCAL, EVENT_EDITOR_DIALOG, AdwWindow);
 
 GtkWidget*           gcal_event_editor_dialog_new                (void);