Avoid manual handling of icons in request field list items

* Use `PurpleKeyValuePair` as field list items
* Replace `icons` list with `has_icons` flag, thus fixing memleak
* Replace `purple_request_field_list_get_icons` with `purple_request_field_list_has_icons`

Testing Done:
Compile and run.

Reviewed at https://reviews.imfreedom.org/r/563/