widget: Change callback scope

widget: Change callback scope

Look who changed his mind since commit 8e2ffb3b46 :-)

The "call" scope means that the callback is only used during the
function call itself (here: gtk_widget_class_install_action()).

That's clearly wrong here, as the callback is invoked every time
the action is activated.

Arguably the "notified" scope is a better match here, where the
lack of a GDestroyNotify parameter suggests that the callback may
be used forever (which is the case here).

Related: #3498