API: accessible: Add widget_set and widget_unset vfuncs

API: accessible: Add widget_set and widget_unset vfuncs

I expect them to be used a lot, so this approach seems better than
requiring signals that connect to "notify::widget". Also, we can't use
regular functions (like dispose or constructed), becaiuse those assume
that (un)setting of the widget only happens once and with the current
design (a puble set_widget() function) we can't really guarantee that.

Also, I split them into two separate functions as one function is part
of construction and the other part of destruction of the object. And it
doesn't sound like a good idea to have that both be part of one
function.