to match gimp_int_combo_box_new(), and add gimp_int_store_new_valist().

Move the va_list of (name, value) parsing code to
gimp_int_store_new_valist() and use it from
gimp_int_combo_box_new_valist().

This makes the entire GimpIntStore/GimpIntComboBox stuff useable much
more generically for the price of an incompatible change of a public
function that is used exactly once outside of libgimpwidgets.