Use g_signal_connect_object for adjustments in GtkSpinButtonAccessible

Use g_signal_connect_object for adjustments in GtkSpinButtonAccessible

An instance of GtkAdjustment may be used by another instance after
the spin button widget is destroyed. In that case, the function
gtk_spin_button_accessible_value_changed() will be called with an
invalid argument. This situation is often caused when one use
GtkCellRendererSpin widget. To avoid invalid call of the function,
the signal handler for the "value-changed" signal should be disconnected
when the spin-button widget is destroyed.
Using g_signal_connect_object achieves just that.
https://bugzilla.gnome.org/show_bug.cgi?id=691592