a11y: add the possibility to set the accessible object of a widget

In the common case, the accessible object is created by the
own widget. In some cases it is needed to specify a custom
accessible, as some of the logic will be implemented on the
javascript code (extend functionality using Components vs Hierarchy).

https://bugzilla.gnome.org/show_bug.cgi?id=648623
a11y: add a generic accessible

Using the same idea that shell-generic-container. It implements
AtkValue with a dummy implementation based on signals. Javascript
code would connect to that and returns the proper value.

https://bugzilla.gnome.org/show_bug.cgi?id=648623
a11y: using generic accessible at slider

Needed in order to fill the AtkValue implementation using
signal callbacks (ala ShellGenericContainer).

https://bugzilla.gnome.org/show_bug.cgi?id=648623