Remove all uses of GtkTreeView from Orca

GtkTreeView is deprecated and not accessible in GTK4. We currently use GTK3, so no immediate danger.

Orca's preferences dialog will get a makeover. Yay!

But we should also modernize Orca's other uses of GtkTreeView that are outside of the Preferences dialog, e.g.

• The commands-list viewer in LearnModePresenter
• The list of notifications in NotificationPresenter
• The list of elements by type (headings, links, etc.) in OrcaNavListGUI

We can either do this independently using other accessible options that are present in both GTK3 and GTK4, or wait and do it as part of the Preferences work.

Screenshots have been attached as comments.

Assigning to myself for now to see if we can do something that is both compatible in GTK3 and GTK4.