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.
Make text change notification work for editables, by connecting
to the ::insert-text and ::delete-text signals on the wrapped
GtkText widget, and for GtkTextView by connecting to the
corresponding GtkTextBuffer signals.
This code is more or less directly copied from GtkTextViewAccessible
and GtkEntryAccessible in GTK 3.
