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/
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"Project-Id-Version: Pidgin\n"
"Report-Msgid-Bugs-To: \n"
"POT-Creation-Date: 2021-01-12 00:20-0600\n"
"PO-Revision-Date: 2017-09-19 23:45+0000\n"
"Last-Translator: Richard Laager <rlaager@wiktel.com>\n"