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 GtkContainerAccessible innards
from the GtkMenuItemAccessible implementation.
The remove handler would not check that removed widgets do indeed have
an accessible associated with them and would happily run all the removal
code with a NULL object. Not good.
