When gtk_accel_label_set_accel_object() is called on a GtkAccelLabel, the
label should be fetching the accelerators on that object.

It doesn't now, which means that if no accelerators are set on the object
being watched by the the GtkAccelLabel, it won't show the accelerators that
were set prior to set_accel_object().

Also, if the accel object is unset (passing NULL to set_accel_object()),
the label will continue to show the last state of the previous accel
object.  I haven't verified this behaviour though.

The simple fix is to add a call to gtk_accel_label_refetch() in
gtk_accel_label_set_accel_object().  Patch to do this is attached below.