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.
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.
