libgimpwidgets: add a "label" property and API to GimpIntComboBox

If set, the label is displayed left-aligned inside the combo box, and
the normal content moves to the right. Reconfigure the combo's
contents when the popup is shown/hidden, so the popup widget is not
affected by the label. This requires an evil hack because of a bug in
GtkCellView. The hack automatically disables itself once GTK+ 2.24.19
(which has a fix) is used.