accessible: Add a way to hide accessibles

accessible: Add a way to hide accessibles

Similar to gtk_widget_should_layout(), add a
gtk_accessible_should_present() function that backends can
use to determine whether an accessible should be presented
or not.

Ways to make a widget not presented in a11y:
- hide the widget
- set its role to NONE
- make it have a NULL AT context

We will use this in future to hide the GtkText inside
an entry, since the Text implementation will be done
by the wrapper.