If the accessible object is hidden, we can skip the emission of the
AddAccessible and RemoveAccessible signals on the cache, as those
objects won't be visible in the accessibility tree.
The GtkAtSpiContext is responsible for removing itself from the root,
which will remove itself from the cache. Any code path that leads to the
GtkAtSpiContext instance being collected passes through the
unrealization phase, which will also unregister the context from the
accessibility bus and from the cache.
The AT-SPI cache interface is used to quickly populate the accessible
objects tree.
The tricky bit is ensuring that we emit change notifications on the
cache only when the cache is available, which means waiting until the
root is asynchronously registered.
