a) use GtkTreeStores for GtkTreeViews
b) ironically can't store GtkTreeStore contents in .ui apparently
c) set show_expanders for all non-trees and unconverted cases
d) on-demand subtrees

Change-Id: I3c1036a222daba2c129b1a22ffeb3fe35005ae31
Reviewed-on: https://gerrit.libreoffice.org/63558
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>