libgimpwidgets: UnitStore: Fix conflicting names
gimp_unit_store_get_value()
clashes with the get_value()
method of
its parent class GtkTreeStore. This means trouble for bindings, as seen
here in some pseudocode:
var unit_store = new Gimp.UnitStore();
// Which function are we referencing here, the one from GtkTreeStore or
// the one from GimpUnitStore? Worse, they both have different arguments
unit_store.get_value( ... )