bug 982842 - initial a11y ipc impl r=davidb, bent
bug 1074854 - properly mark proxies in atkobject::accWrap r=davidb

When the accWrap field points at a proxy its value must be the pointer
or'd with IS_PROXY, but we incorrectly stored the plain pointer.
bug 1074862 - more fully initialize DocAccessibleParents r=davidb

Instances of DocAccessibleParent need to behave like any other
ProxyAccessible which means something sensible must happen when we
access these fields of a DocAccessibleParent.