Make GtkScaleButton a widget that has a toggle button
as a child, just like all the other button widgets now.
The immediate benefit of this arrangement is to avoid
the "double focus" problem when we pop up the popup.
Update accessible, demos and tests to match.
This commit exposes the get_type() functions and standard
headers for accessible implementations. This makes it possible
to derive from the GTK accessible implementations without
GType magic tricks. This is necessary, because we require the
a11y type hierarchy to be parallel to the widget type hierarchy.
So, if you derive a widget and need to adjust its a11y implementation,
you have to be able to derive its accessible implementation.
This commit probably exposes more than is absolutely necessary,
it also exposes accessibles of widgets that are unlikely candidates
for deriving from.
This drops the AtkText implementation, and also strips handling
of children out. Instead of listening for enter/leave/press/released,
just listen for state changes on the widget.
