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.
Move instance fields to a private struct, in preparation
for installing a11y headers.
This also required removing access to GtkContainerCellAccessible
innards from the GtkCellAccessible implementation.
I could have tried to make GtkContainerCellAccessible implement
GtkCellAccessibleParent, but the current implementation of that
interface doesn't make sense for it.
