Use const attribute name input in atspi_*_get*_attribute_value()

There is no point in having them mutable, and makes the API look more awkward and harder to use than it actually is.

I can't see any problem with backward compatibility as const gchar* is only more strict, and accepts a gchar*. So this should not have an effect on any existing caller, apart potentially fixing some leaks in auto-generated (binding?) code, or rendering special-cases useless.