into local variables, rather than invoking hide() to delete them.  That way
delete won't be called before hide() actually returned.

(on_about_dialog_hide): Don't use std::auto_ptr<>::reset() but transfer
ownership into local variables.  I had a look at the libstdc++-v3 headers and
it gave me the creeps -- reset() deletes _before_ assigning the new pointer.

(on_preferences): ditto.