Instead rely on state_flags & GTK_STATE_FLAG_INSENSITIVE to tell us if a
widget is sensitive.
This has the huge benefit that the way the widget is actually rendered
corresponds to the return value of gtk_widget_is_sensitive().

As a side effect, we do not ever allow unsetting the
GTK_STATE_FLAG_INSENSITIVE for a widget the is set to not be sensitive
(via gtk_widget_set_sensitive()). This way we stop propagation of making
stuff sensitive at insensitive widgets.

https://bugzilla.gnome.org/show_bug.cgi?id=642918