The Selection interface is defined in terms of child
positions, so we need to always translate from that
to model positions if we want to use the selection
model apis.
This implementation works for both GtkListView and
GtkGridView, and by extension, also for GtkColumnView
and GtkDropDown, since those just use a list view
internally.
This is a bit different from the way things were done
in GTK 3 - we follow what was done for GtkStackSwitcher,
and make the tab bar carry the GTK_ACCESSIBLE_TAB_LIST
role, and implement Selection there.
This requires some cleanup to remove assumptions
about accessibles being widgets in the backend,
and some code to navigate the tree with these
extra objects in between widgets.
The accessibles for stack pages have the role
GTK_ACCESSIBLE_ROLE_TAB_PANEL. This is the first
step towards implementing the tabs patterns
as described in the aria authoring guidelines
for GtkStack.
