Replaces GncOptionValue<GncMultiChoiceOptionChoices> because having the
vector as the value obviously wouldn't work and besides it needs
additional functions.
The objective of the free functions is to hide the GncOption from language
bindings so that the GncOptions can be moved into the GncOptionDB instead
of having shared ptrs splattered around the heap. Nearly all access to
the options can then be mediated through the GncOptionDB container.
Note that gnc_option_db_new creates the GncOptionDB on the heap and
returns a raw ptr, so it's up to the creator of the GncOptionDB to
call delete on it when it's no longer needed.
