... leading to a crash

Add gimp_data_is_copyable() and gimp_data_is_dulicatable().

Use gimp_data_is_duplicatable() when setting the sensitivity of the
various "foo-duplicate" actions, instead of inspecting the object's
GimpDataClass::duplicate pointer directly, since this is no longer
an indication of whether a GimpData object is duplicatable or not
(since commit 33de4d55).