PopplerFormField: add getter for alternative ui name

There are pdf files which carry a tooltip string in this field, so applications need this getter to show it. API added is:

gchar*
poppler_form_field_get_alternate_ui_name (PopplerFormField *field)

Related Poppler issue #34 (closed)

Related Evince issue: https://gitlab.gnome.org/GNOME/evince/issues/842