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.c

@@ -48,7 +48,7 @@
 
 struct _GcalEventEditorDialog
 {
-  GtkDialog         parent;
+  AdwWindow               parent;
 
   GtkWidget              *cancel_button;
   GtkWidget              *delete_button;
@@ -89,7 +89,7 @@ static void          on_calendar_selected_action_cb              (GSimpleAction
                                                                   GVariant           *value,
                                                                   gpointer            user_data);
 
-G_DEFINE_TYPE (GcalEventEditorDialog, gcal_event_editor_dialog, GTK_TYPE_DIALOG)
+G_DEFINE_TYPE (GcalEventEditorDialog, gcal_event_editor_dialog, ADW_TYPE_WINDOW)
 
 enum
 {
@@ -559,9 +559,6 @@ gcal_event_editor_dialog_constructed (GObject* object)
 
   gtk_window_set_title (GTK_WINDOW (object), "");
 
-  /* titlebar */
-  gtk_window_set_titlebar (GTK_WINDOW (object), self->titlebar);
-
   /* Actions */
   self->action_group = g_simple_action_group_new ();
   g_action_map_add_action_entries (G_ACTION_MAP (self->action_group),