GtkSpinButton: Always emit value-changed when the adjustment is changed

GtkSpinButton: Always emit value-changed when the adjustment is changed

Changing adjustment via the property setter would not emit
value-changed, however changing it via gtk_spin_button_configure would.

This inconsistency had the following side-effects:

  - Setting an adjustment with a different value would not update the
    value shown by the spin button.

  - Creating a spin button like this (common in GtkBuilder XML) will
    not show the initial value:
      g_object_new (GTK_TYPE_SPIN_BUTTON, "adjustment", adj, NULL);

Let's use the same code path (ie. gtk_spin_button_configure) for all
public facing API for setting adjustment. The code that handled the
details of swapping out the old adjustment with the new has been split
into an unset_adjustment method and the rest has been folded into
gtk_spin_button_configure.

A spin button really needs an adjustment to work, so we don't need
most of the NULL checks. However we do need to check in
unset_adjustment because setting a new adjustment during object
creation might try to unset a non-existent one.

https://bugzilla.gnome.org/show_bug.cgi?id=734660