Clean up Finch preferences page

Instead of mixing two strings into a `GList`, use a `PurpleKeyValuePair`.
Use struct initializers and `G_N_ELEMENTS`.
Rename `lv` to `choices` for clarity.
Change from `PurpleRequestFieldList` to `PurpleRequestFieldChoice` as we don't need multiple selection.

Testing Done:
Opened Finch, opened Preferences, saved them, and exited Finch. Re-opened Prefs and confirmed they were saved and as expected.

Reviewed at https://reviews.imfreedom.org/r/2416/