Fix accessibility not reporting changes

[m-sasha]

sun.lwawt.macosx.CAccessible does this weird thing:

    private CAccessible(final Accessible accessible) {
        ...
        if (accessible instanceof Component) {
            addNotificationListeners((Component)accessible);
        }
    }

    public void addNotificationListeners(Component c) {
        if (c instanceof Accessible) {
            AccessibleContext ac = ((Accessible)c).getAccessibleContext();
            ac.addPropertyChangeListener(new AXChangeNotifier());
        }
    }

So unless the Accessible is an instance of java.awt.Component, it will not register its property change listener with its accessible context. This prevents us from notifying it when a value changes (a progress bar updates for example).

Proposed Changes

Make ComposeAccessible a subclass of java.awt.Component.

This also requires renaming its accessibleContext property, because java.awt.Component already has a protected field with the same name.

Testing

Test: Can't test this without exposing something about the amount of property change listeners registered with ComposeAccessibleComponent. Tested manually with a progress bar (needs a bit of additional code, which I'll add in the next PR).

[Walingar]

LGTM, checked on MacOs it works fine. Didn't notice regressions. Still need to check on Windows