Reference count struct dictionary.

Reference count struct dictionary.

This change:

* Adds a ref count to struct dictionary.
* Wraps dict_destroy in a ref-counted wrapper dict_unref.
* Replaces most calls to dict_destroy with dict_unref.
* Adds a new function dict_ref.
* Changes PsppireDict so take a reference to the struct dict it contains.
* Fixes some leaked dictionaries in PsppireImportAssistant.