This commit exposes the get_type() functions and standard
headers for accessible implementations. This makes it possible
to derive from the GTK accessible implementations without
GType magic tricks. This is necessary, because we require the
a11y type hierarchy to be parallel to the widget type hierarchy.
So, if you derive a widget and need to adjust its a11y implementation,
you have to be able to derive its accessible implementation.
This commit probably exposes more than is absolutely necessary,
it also exposes accessibles of widgets that are unlikely candidates
for deriving from.
The way submenus are handled here isn't really right, since
they can come and go at runtime, but I've left it like this
for now. A side-effect is that we can't use
gtk_widget_class_set_accessible_type here, but have to
override get_accessible.
