Fix tab switching for Gtk4 compatibility
The migration to Gtk4 introduced changes in the widget hierarchy, which
prevented DialogBase from finding its parent Gtk::Notebook using the
get_parent() method. This issue affected the tab switching functionality,
where the UI failed to highlight or switch to the dialog's tab when
activated.
This patch updated the parent widget retrieval method to
get_parent()->get_parent(). This adjustment resolves the issue where
tab switching failed to activate and highlight the correct tab in Gtk4,
ensuring consistent behavior as expected.