We are doing too much work during the construction phase of the
AT-SPI backend for GtkATContext. Instead of having the AtSpiContext
register itself at construction time, let's add explicit realize
and unrealize operations, and connect the ATContext realization to the
rooting operation of a GtkWidget.
Instead of monitoring the list of toplevels, rely
on GtkWindow updating the HIDDEN state before windows
get removed. This is better, since we still have the
object available when it happens, so we can pass it
to the ATs.
We need to translate the list model position into
an accessible tree position, since hidden toplevels
will be skipped.
While we are at it, add an api for this notification
that will be used in the next commit.
GtkAtSpiRoot is not a context, which means it needs to emit
ChildrenChanged events by itself whenever a toplevel is added to, or
removed from, the list of toplevels.
It turns out that accerciser depends on this undocumented
method that is not in the xml at all, otherwise interface
sections in the accerciser ui never get enabled.
When we create the first AT-SPI context we also need to register the
accessible root on the accessibility bus. The accessible root object is
the main entry point of an accessible application, and it holds the
global state to present to the ATs that connect to the bus.
